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Merge pull request #8197 from fredlf/adding_official-repo-guidelines

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Docs for Official Repo release
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Fred Lifton 2014-09-23 20:55:07 -07:00
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@ -65,6 +65,7 @@ pages:
- ['docker-hub/accounts.md', 'Docker Hub', 'Accounts']
- ['docker-hub/repos.md', 'Docker Hub', 'Repositories']
- ['docker-hub/builds.md', 'Docker Hub', 'Automated Builds']
- ['docker-hub/official_repos.md', 'Docker Hub', 'Official Repo Guidelines']
# Examples:
- ['examples/index.md', '**HIDDEN**']
@ -83,17 +84,18 @@ pages:
- ['articles/networking.md', 'Articles', 'Advanced networking']
- ['articles/security.md', 'Articles', 'Security']
- ['articles/https.md', 'Articles', 'Running Docker with HTTPS']
- ['articles/host_integration.md', 'Articles', 'Automatically starting Containers']
- ['articles/host_integration.md', 'Articles', 'Automatically starting containers']
- ['articles/baseimages.md', 'Articles', 'Creating a base image']
- ['articles/dockerfile_best-practices.md', 'Articles', 'Best practices for writing Dockerfiles']
- ['articles/certificates.md', 'Articles', 'Using certificates for repository client verification']
- ['articles/using_supervisord.md', 'Articles', 'Using Supervisor']
- ['articles/cfengine_process_management.md', 'Articles', 'Process management with CFEngine']
- ['articles/puppet.md', 'Articles', 'Using Puppet']
- ['articles/chef.md', 'Articles', 'Using Chef']
- ['articles/dsc.md', 'Articles', 'Using PowerShell DSC']
- ['articles/ambassador_pattern_linking.md', 'Articles', 'Cross-Host linking using Ambassador Containers']
- ['articles/ambassador_pattern_linking.md', 'Articles', 'Cross-Host linking using ambassador containers']
- ['articles/runmetrics.md', 'Articles', 'Runtime metrics']
- ['articles/baseimages.md', 'Articles', 'Creating a Base Image']
- ['articles/b2d_volume_resize.md', 'Articles', 'Increasing a Boot2Docker Volume']
- ['articles/b2d_volume_resize.md', 'Articles', 'Increasing a Boot2Docker volume']
# Reference
- ['reference/index.md', '**HIDDEN**']

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@ -55,5 +55,14 @@ image to base your new minimal containers `FROM`:
COPY true-asm /true
CMD ["/true"]
The Dockerfile above is from extremely minimal image - [tianon/true](
The `Dockerfile` above is from an extremely minimal image - [tianon/true](
https://github.com/tianon/dockerfiles/tree/master/true).
## More resources
There are lots more resources available to help you write your 'Dockerfile`.
* There's a [complete guide to all the instructions](/reference/builder/) available for use in a `Dockerfile` in the reference section.
* To help you write a clear, readable, maintainable `Dockerfile`, we've also
written a [`Dockerfile` Best Practices guide](/articles/dockerfile_best-practices).
* If you're working on an Official Repo, be sure to check out the [Official Repo Guidelines](/docker-hub/official_repos/).

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page_title: Best Practices for Writing Dockerfiles
page_description: Hints, tips and guidelines for writing clean, reliable Dockerfiles
page_keywords: Examples, Usage, base image, docker, documentation, dockerfile, best practices, hub, official repo
# Best Practices for Writing `Dockerfile`s
## Overview
Docker can build images automatically by reading the instructions from a
`Dockerfile`, a text file that contains all the commands, in order, needed to
build a given image. `Dockerfile`s adhere to a specific format and use a
specific set of instructions. You can learn the basics on the
[Dockerfile Reference](https://docs.docker.com/reference/builder/) page. If
you’re new to writing `Dockerfile`s, you should start there.
This document covers the best practices and methods recommended by Docker,
Inc. and the Docker Community for creating easy-to-use, effective
`Dockerfile`s. We strongly suggest you follow these recommendations (in fact,
if you’re creating an Official Image, you *must* adhere to these practices).
You can see many of these practices and recommendations in action in the [buildpack-deps `Dockerfile`](https://github.com/docker-library/buildpack-deps/blob/master/jessie/Dockerfile).
> Note: for more detailed explanations of any of the Dockerfile commands
>mentioned here, visit the [Dockerfile Reference](https://docs.docker.com/reference/builder/) page.
## General Guidelines and Recommendations
### Containers should be ephemeral
The container produced by the image your `Dockerfile` defines should be as
ephemeral as possible. “Ephemeral” here means that it can be stopped and
destroyed and a new one built and put in place with an absolute minimum of
set-up and configuration.
### Use a [`.dockerignore` file](https://docs.docker.com/reference/builder/#the-dockerignore-file)
For faster uploading and efficiency during `docker build`, you should make use
of a `.dockerignore` file to exclude files or directories from the build
context and final image. For example, unless`.git` is needed by your build
process or scripts, you should add it to `.dockerignore`, which can save many
megabytes worth of upload time.
### Avoid installing unnecessary packages
You should avoid installing extra or unnecessary packages just because they
might be “nice to have.” For example, you don’t need to include a text editor
in a database image.
### Run only one process per container
In almost all cases, you should only run a single process in a single
container. Decoupling applications into multiple containers makes it much
easier to scale horizontally and reuse containers. If that service depends on
another service, make use of [container linking](https://docs.docker.com/userguide/dockerlinks/).
### Minimize the number of layers
You need to find the balance between readability (and thus long-term
maintainability) of the `Dockerfile` and minimizing the number of layers it
uses. Be strategic and cautious about the number of layers you use.
### Sort multi-line arguments
Whenever possible, ease later changes by sorting multi-line arguments
alphanumerically. This will help you avoid duplication of packages and make the
list much easier to update. This also makes PRs a lot easier to read and
review. Adding a space before a backslash (`\`) helps as well.
Here’s an example from the [`buildpack-deps` image](https://github.com/docker-library/buildpack-deps):
RUN apt-get update && apt-get install -y \
bzr \
cvs \
git \
mercurial \
subversion
## The `Dockerfile` instructions
This section contains specific recommendations for the correct usage of the
various instructions contained in a `Dockerfile`.
### [`FROM`](https://docs.docker.com/reference/builder/#from)
Whenever possible, use Official Repositories as the basis for your image. We
recommend the [Debian image](https://registry.hub.docker.com/_/debian/) since
it’s very tightly controlled and kept extremely minimal (currently under 100
mb), while still being a full distribution.
### [`RUN`](https://docs.docker.com/reference/builder/#run)
As always, to make your `Dockerfile` more readable, understandable, and
maintainable, put long or complex `RUN` statements on multiple lines separated with
backslashes.
Probably the most common use-case for `RUN` is an application of `apt-get`.
When using `apt-get`, here a few things to keep in mind:
* Don’t do `RUN apt-get update` on a single line. This will cause
caching issues if the referenced archive gets updated, which will make your
subsequent `apt-get install` fail without comment.
* For the most part, to keep your code more readable and maintainable, avoid
`RUN apt-get install -y package-foo && apt-get install -y package-bar`.
* Avoid `RUN apt-get upgrade` or `dist-upgrade`, since many of the “essential”
packages from the base images will fail to upgrade inside an unprivileged
container. If a base package is out of date, you should contact its
maintainers. If you know there’s a particular package, `foo`, that needs to be
updated, use `apt-get install -y foo` and it will update automatically.
* Do use `RUN apt-get update && apt-get install -y package-bar package-foo
package-baz`. Writing the instruction this way not only makes it easier to read
and maintain, but also, by including `apt-get update`, ensures that the cache
will naturally be busted and the latest versions will be installed with no
further coding or manual intervention required.
* Further natural cache-busting can be realized by version-pinning packages
(e.g., `package-foo=1.3.*`). This will force retrieval of that version
regardless of what’s in the cache.
Forming your `apt-get` code this way will greatly ease maintenance and reduce
failures due to unanticipated changes in required packages.
#### Example
Below is a well-formed `RUN` instruction that demonstrates the above
recommendations. Note that the last package, `s3cmd`, specifies a version
`1.1.0*`. If the image previously used an older version, specifying the new one
will cause a cache bust of `apt-get update` and ensure the installation of
the new version (which in this case had a new, required feature).
RUN apt-get update && apt-get install -y \
aufs-tools \
automake \
btrfs-tools \
build-essential \
curl \
dpkg-sig \
git \
iptables \
libapparmor-dev \
libcap-dev \
libsqlite3-dev \
lxc=1.0* \
mercurial \
parallel \
reprepro \
ruby1.9.1 \
ruby1.9.1-dev \
s3cmd=1.1.0*
### [`CMD`](https://docs.docker.com/reference/builder/#cmd)
The `CMD` instruction should be used to run the software contained by your
image, along with any arguments. `CMD` should almost always be used in the
form of `CMD [“executable”, “param1”, “param2”…]`. Thus, if the image is for a
service (Apache, Rails, etc.), you would run something like
`CMD ["apache2","-DFOREGROUND"]`. Indeed, this form of the instruction is
recommended for any service-based image.
In most other cases, `CMD` should be given an interactive shell (bash, python,
perl, etc), for example, `CMD ["perl", "-de0"]`, `CMD ["python"]`, or
`CMD [“php”, “-a”]`. Using this form means that when you execute something like
`docker run -it python`, you’ll get dropped into a usable shell, ready to go.
`CMD` should rarely be used in the manner of `CMD [“param”, “param”]` in
conjunction with [`ENTRYPOINT`](https://docs.docker.com/reference/builder/#entrypoint), unless
you and your expected users are already quite familiar with how `ENTRYPOINT`
works.
### [`EXPOSE`](https://docs.docker.com/reference/builder/#expose)
The `EXPOSE` instruction indicates the ports on which a container will listen
for connections. Consequently, you should use the common, traditional port for
your application. For example, an image containing the Apache web server would
use `EXPOSE 80`, while an image containing MongoDB would use `EXPOSE 27017` and
so on.
For external access, your users can execute `docker run` with a flag indicating
how to map the specified port to the port of their choice.
For container linking, Docker provides environment variables for the path from
the recipient container back to the source (ie, `MYSQL_PORT_3306_TCP`).
### [`ENV`](https://docs.docker.com/reference/builder/#env)
In order to make new software easier to run, you can use `ENV` to update the
`PATH` environment variable for the software your container installs. For
example, `ENV PATH /usr/local/nginx/bin:$PATH` will ensure that `CMD [“nginx”]`
just works.
The `ENV` instruction is also useful for providing required environment
variables specific to services you wish to containerize, such as Postgres’s
`PGDATA`.
Lastly, `ENV` can also be used to set commonly used version numbers so that
version bumps are easier to maintain, as seen in the following example:
ENV PG_MAJOR 9.3
ENV PG_VERSION 9.3.4
RUN curl -SL http://example.com/postgres-$PG_VERSION.tar.xz | tar -xJC /usr/src/postgress && …
ENV PATH /usr/local/postgres-$PG_MAJOR/bin:$PATH
Similar to having constant variables in a program (as opposed to hard-coding
values), this approach lets you change a single `ENV` instruction to
auto-magically bump the version of the software in your container.
### [`ADD`](https://docs.docker.com/reference/builder/#add) or [`COPY`](https://docs.docker.com/reference/builder/#copy)
Although `ADD` and `COPY` are functionally similar, generally speaking, `COPY`
is preferred. That’s because it’s more transparent than `ADD`. `COPY` only
supports the basic copying of local files into the container, while `ADD` has
some features (like local-only tar extraction and remote URL support) that are
not immediately obvious. Consequently, the best use for `ADD` is local tar file
auto-extraction into the image, as in `ADD rootfs.tar.xz /`.
Because image size matters, using `ADD` to fetch packages from remote URLs is
strongly discouraged; you should use `curl` or `wget` instead. That way you can
delete the files you no longer need after they’ve been extracted and you won't
have to add another layer in your image. For example, you should avoid doing
things like:
ADD http://example.com/big.tar.xz /usr/src/things/
RUN tar -xJf /usr/src/things/big.tar.xz -C /usr/src/things
RUN make -C /usr/src/things all
And instead, do something like:
RUN mdkir -p /usr/src/things \
&& curl -SL http://example.com/big.tar.gz \
| tar -xJC /usr/src/things \
&& make -C /usr/src/things all
For other items (files, directories) that do not require `ADD`’s tar
auto-extraction capability, you should always use `COPY`.
### [`ENTRYPOINT`](https://docs.docker.com/reference/builder/#entrypoint)
The best use for `ENTRYPOINT` is as a helper script. Using `ENTRYPOINT` for
other tasks can make your code harder to understand. For example,
`docker run -it official-image bash` is much easier to understand than
`docker run -it --entrypoint bash official-image -i`, especially for Docker
beginners.
In order to avoid a situation where commands are run without clear visibility
to the user, make sure your script ends with something like `exec "$@"`. After
the entrypoint completes, the script will transparently bootstrap the command
invoked by the user, making what has been run clear to the user (for example,
`docker run -it mysql mysqld --some --flags` will transparently run
`mysqld --some --flags` after `ENTRYPOINT` runs `initdb`).
For example, let’s look at the `Dockerfile` for the
[Postgres Official Image](https://github.com/docker-library/postgres).
It refers to the following script:
```bash
#!/bin/bash
set -e
if [ "$1" = 'postgres' ]; then
chown -R postgres "$PGDATA"
if [ -z "$(ls -A "$PGDATA")" ]; then
gosu postgres initdb
fi
exec gosu postgres "$@"
fi
exec "$@"
```
That script then gets copied into the container and the run via `ENTRYPOINT` on
container startup:
COPY ./docker-entrypoint.sh /
ENTRYPOINT ["/docker-entrypoint.sh"]
### [`VOLUME`](https://docs.docker.com/reference/builder/#volume)
The `VOLUME` instruction should be used to expose any database storage area,
configuration storage, or files/folders created by your docker container. You
are strongly encouraged to use `VOLUME` for any mutable and/or user-serviceable
parts of your image.
### [`USER`](https://docs.docker.com/reference/builder/#user)
If a service can run without privileges, use `USER` to change to a non-root
user. Start by creating the user and group in the `Dockerfile` with something
like `RUN groupadd -r postgres && useradd -r -g postgres postgres`.
>**Note** that users/groups in an image get assigned a non-deterministic
>UID/GID in that the “next” UID/GID gets assigned regardless of image
>rebuilds. So, if it’s critical, you should assign an explicit UID/GID.
You should avoid installing or using `sudo` since it has unpredictable TTY and
signal-forwarding behavior that can cause more more problems than it solves. If
you absolutely need functionality similar to `sudo` (e.g., initializing the
daemon as root but running it as non-root), you may be able to use
[“gosu”](https://github.com/tianon/gosu).
Lastly, to reduce layers and complexity, try to minimize switching `USER` back
and forth frequently.
### [`WORKDIR`](https://docs.docker.com/reference/builder/#workdir)
For clarity and reliability, you should always use absolute paths for your
`WORKDIR`. Also, you should use `WORKDIR` instead of proliferating
instructions like `RUN cd … && do-something`, which are hard to read,
troubleshoot, and maintain.
### [`ONBUILD`](https://docs.docker.com/reference/builder/#onbuild)
`ONBUILD` is only useful for images that are going to be built `FROM` a given
image. For example, you would use `ONBUILD` for a language stack image that
builds arbitrary user software written in that language within the
`Dockerfile`, as you can see in [Ruby’s `ONBUILD` variants](https://github.com/docker-library/ruby/blob/master/2.1/onbuild/Dockerfile).
Images built from `ONBUILD` should get a separate tag, for example:
`ruby:1.9-onbuild` or `ruby:2.0-onbuild`.
Be careful when putting `ADD` or `COPY` in `ONBUILD`. The “onbuild” image will
fail catastrophically if the new build's context is missing the resource being
added. Adding a separate tag, as recommended above, will help mitigate this by
allowing the `Dockerfile` author to make a choice.
## Examples For Official Repositories
These Official Repos have exemplary `Dockerfile`s:
* [Go](https://registry.hub.docker.com/_/golang/)
* [Perl](https://registry.hub.docker.com/_/perl/)
* [Hy](https://registry.hub.docker.com/_/hylang/)
* [Rails](https://registry.hub.docker.com/_/rails)
## Additional Resources:
* [Dockerfile Reference](https://docs.docker.com/reference/builder/#onbuild)
* [More about Base Images](https://docs.docker.com/articles/baseimages/)
* [More about Automated Builds](https://docs.docker.com/docker-hub/builds/)
* [Guidelines for Creating Official Repositories](https://docs.docker.com/docker-hub/official_repos/)

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page_title: Guidelines for Official Repositories on Docker Hub
page_description: Guidelines for Official Repositories on Docker Hub
page_keywords: Docker, docker, registry, accounts, plans, Dockerfile, Docker Hub, docs, official, image, documentation
# Guidelines for Creating and Documenting Official Repositories
## Introduction
You’ve been given the job of creating an image for an Official Repository
hosted on [Docker Hub Registry](https://registry.hub.docker.com/). These are
our guidelines for getting that task done. Even if you’re not
planning to create an Official Repo, you can think of these guidelines as best
practices for image creation generally.
This document consists of two major sections:
* A list of expected files, resources and supporting items for your image
* Examples embodying those practices
## Expected Files & Resources
### A Git repository
Your image needs to live in a Git repository, preferably on GitHub. (If you’d
like to use a different provider, please [contact us](mailto:feedback@docker.com)
directly.) Docker **strongly** recommends that this repo be publicly
accessible.
If the repo is private or has otherwise limited access, you must provide a
means of at least “read-only” access for both general users and for the
docker-library maintainers, who need access for review and building purposes.
### A `Dockerfile`
Complete information on `Dockerfile`s can be found in the [Reference section](https://docs.docker.com/reference/builder/).
We also have a page discussing [best practices for writing `Dockerfile`s](/articles/dockerfile_best-practices).
Your `Dockerfile` should adhere to the following:
* It must be written either by using `FROM scratch` or be based on another,
established Official Image.
* It must follow `Dockerfile` best practices. These are discussed in the
[Best Practices document](/articles/dockerfile_best-practices). In addition,
Docker engineer Michael Crosby has a good discussion of Dockerfiles in
this [blog post](http://crosbymichael.com/dockerfile-best-practices-take-2.html).
While [`ONBUILD` triggers](https://docs.docker.com/reference/builder/#onbuild)
are not required, if you choose to use them you should:
* Build both `ONBUILD` and non-`ONBUILD` images, with the `ONBUILD` image
built `FROM` the non-`ONBUILD` image.
* The `ONBUILD` image should be specifically tagged, for example, `ruby:
latest`and `ruby:onbuild`, or `ruby:2` and `ruby:2-onbuild`.
### A short description
Include a brief description of your image (in plaintext). Only one description
is required; you don’t need additional descriptions for each tag. The file
should also:
* Be named `README-short.txt`
* Reside in the repo for the “latest” tag
* Not exceed 200 characters.
### A logo
Include a logo of your company or the product (png format preferred). Only one
logo is required; you don’t need additional logo files for each tag. The logo
file should have the following characteristics:
* Be named `logo.png`
* Should reside in the repo for the “latest” tag
* Should be 200px min. in one dimension, 200px max. in the other.
* Square or wide (landscape) is preferred over tall (portrait), but exceptions
can be made based on the logo needed.
### A long description
Include a comprehensive description of your image (in markdown format). Only
one description is required; you don’t need additional descriptions for each
tag. The file should also:
* Be named `README.md`
* Reside in the repo for the “latest” tag
* Be no longer than absolutely necessary, while still addressing all the
content requirements.
In terms of content, the long description must include the following sections:
* Overview & Links
* How-to/Usage
* Issues & Contribution Info
#### Overview & links
This section should provide:
* an overview of the software contained in the image, similar to the
introduction in a Wikipedia entry,
* a selection of links to outside resources that help to describe the software,
* a *mandatory* link to the `Dockerfile`.
#### How-to/usage
A section that describes how to run and use the image, including common use
cases and example `Dockerfile`s (if applicable). Try to provide clear,step-by-
step instructions wherever possible.
##### Issues & Contribution Info
In this part, point users to any resources that can help them contribute to the
project. Include contribution guidelines and any specific instructions related
to your development practices. Include a link to
[Docker’s resources for contributors](https://docs.docker.com/contributing/contributing/).
Be sure to include contact info, handles, etc. for official maintainers.
##### Issues
Include a brief section letting users know where they can go for help and how
they can file issues with the repo. Point them to any specific IRC channels,
issue trackers, contacts, additional “how-to” information or other resources.
### License
Include a file, `LICENSE`, of any applicable license. Docker recommends using
the license of the software contained in the image, provided it allows Docker,
Inc. to legally build and distribute the image. Otherwise Docker recommends
adopting the [Expat license](http://directory.fsf.org/wiki/License:Expat).
## Examples
Below are sample short and long description files for an imaginary image
containing Ruby on Rails.
### Short description
`README-short.txt`
`Ruby on Rails is an open-source application framework written in Ruby. It emphasizes best practices such as convention over configuration, active record pattern, and the model-view-controller pattern.`
### Long description
`README.md`
```markdown
# What is Ruby on Rails
Ruby on Rails, often simply referred to as Rails, is an open source web application framework which runs via the Ruby programming language. It is a full-stack framework: it allows creating pages and applications that gather information from the web server, talk to or query the database, and render templates out of the box. As a result, Rails features a routing system that is independent of the web server.
> [wikipedia.org/wiki/Ruby_on_Rails](https://en.wikipedia.org/wiki/Ruby_on_Rails)
# How to use this image
## Create a `Dockerfile` in your rails app project
FROM rails:onbuild
Put this file in the root of your app, next to the `Gemfile`.
This image includes multiple `ONBUILD` triggers so that should be all that you need for most applications. The build will `ADD . /usr/src/app`, `RUN bundle install`, `EXPOSE 3000`, and set the default command to `rails server`.
Then build and run the docker image.
docker build -t my-rails-app .
docker run --name some-rails-app -d my-rails-app
Test it by visiting `http://container-ip:3000` in a browser. On the other hand, if you need access outside the host on port 8080:
docker run --name some-rails-app -p 8080:3000 -d my-rails-app
Then hit `http://localhost:8080` or `http://host-ip:8080` in a browser.
```
For more examples, take a look at these repos:
* [Go](https://github.com/docker-library/golang)
* [PostgreSQL](https://github.com/docker-library/postgres)
* [Buildpack-deps](https://github.com/docker-library/buildpack-deps)
* ["Hello World" minimal container](https://github.com/docker-library/hello-world)
* [Node](https://github.com/docker-library/node)

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@ -11,6 +11,11 @@ Docker image. By calling `docker build` from your terminal, you can have
Docker build your image step by step, executing the instructions
successively.
This page discusses the specifics of all the instructions you can use in your
`Dockerfile`. To further help you write a clear, readable, maintainable
`Dockerfile`, we've also written a [`Dockerfile` Best Practices guide](/articles/dockerfile_best-practices).
## Usage
To [*build*](../commandline/cli/#cli-build) an image from a source repository,

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root@8196968dac35:/#
> **Note:**
> This is just the briefest introduction to creating images. We've
> This is just a brief introduction to creating images. We've
> skipped a whole bunch of other instructions that you can use. We'll see more of
> those instructions in later sections of the Guide or you can refer to the
> [`Dockerfile`](/reference/builder/) reference for a
> detailed description and examples of every instruction.
> To help you write a clear, readable, maintainable `Dockerfile`, we've also
> written a [`Dockerfile` Best Practices guide](/articles/dockerfile_best-practices).
## Setting tags on an image