moby/docs/sources/reference/builder.rst

:title: Dockerfile Reference
:description: Dockerfiles use a simple DSL which allows you to automate the steps you would normally manually take to create an image.
:keywords: builder, docker, Dockerfile, automation, image creation

.. _dockerbuilder:

====================
Dockerfile Reference
====================

**Docker can act as a builder** and read instructions from a text
``Dockerfile`` to automate the steps you would otherwise take manually
to create an image. Executing ``docker build`` will run your steps and
commit them along the way, giving you a final image.

.. contents:: Table of Contents

.. _dockerfile_usage:

1. Usage
========

To :ref:`build <cli_build>` an image from a source repository, create
a description file called ``Dockerfile`` at the root of your
repository. This file will describe the steps to assemble the image.

Then call ``docker build`` with the path of your source repository as
argument (for example, ``.``):

    ``sudo docker build .``

The path to the source repository defines where to find the *context*
of the build. The build is run by the Docker daemon, not by the CLI,
so the whole context must be transferred to the daemon. The Docker CLI
reports "Uploading context" when the context is sent to the daemon.

You can specify a repository and tag at which to save the new image if the
build succeeds:

    ``sudo docker build -t shykes/myapp .``

The Docker daemon will run your steps one-by-one, committing the
result to a new image if necessary, before finally outputting the 
ID of your new image. The Docker daemon will automatically clean 
up the context you sent.

Note that each instruction is run independently, and causes a new image 
to be created - so ``RUN cd /tmp`` will not have any effect on the next
instructions.

Whenever possible, Docker will re-use the intermediate images, 
accelerating ``docker build`` significantly (indicated by ``Using cache``:

.. code-block:: bash

   $ docker build -t SvenDowideit/ambassador .
   Uploading context 10.24 kB
   Uploading context 
   Step 1 : FROM docker-ut
    ---> cbba202fe96b
   Step 2 : MAINTAINER SvenDowideit@home.org.au
    ---> Using cache
    ---> 51182097be13
   Step 3 : CMD env | grep _TCP= | sed 's/.*_PORT_\([0-9]*\)_TCP=tcp:\/\/\(.*\):\(.*\)/socat TCP4-LISTEN:\1,fork,reuseaddr TCP4:\2:\3 \&/'  | sh && top
    ---> Using cache
    ---> 1a5ffc17324d
   Successfully built 1a5ffc17324d

When you're done with your build, you're ready to look into
:ref:`image_push`.

.. _dockerfile_format:

2. Format
=========

Here is the format of the Dockerfile:

::

    # Comment
    INSTRUCTION arguments

The Instruction is not case-sensitive, however convention is for them to be
UPPERCASE in order to distinguish them from arguments more easily.

Docker evaluates the instructions in a Dockerfile in order. **The
first instruction must be `FROM`** in order to specify the
:ref:`base_image_def` from which you are building.

Docker will treat lines that *begin* with ``#`` as a comment. A ``#``
marker anywhere else in the line will be treated as an argument. This
allows statements like:

::

    # Comment
    RUN echo 'we are running some # of cool things'

.. _dockerfile_instructions:

3. Instructions
===============

Here is the set of instructions you can use in a ``Dockerfile`` for
building images.

.. _dockerfile_from:

3.1 FROM
--------

    ``FROM <image>``

Or

    ``FROM <image>:<tag>``

The ``FROM`` instruction sets the :ref:`base_image_def` for subsequent
instructions. As such, a valid Dockerfile must have ``FROM`` as its
first instruction. The image can be any valid image -- it is
especially easy to start by **pulling an image** from the
:ref:`using_public_repositories`.

``FROM`` must be the first non-comment instruction in the
``Dockerfile``.

``FROM`` can appear multiple times within a single Dockerfile in order
to create multiple images. Simply make a note of the last image id
output by the commit before each new ``FROM`` command.

If no ``tag`` is given to the ``FROM`` instruction, ``latest`` is
assumed. If the used tag does not exist, an error will be returned.

.. _dockerfile_maintainer:

3.2 MAINTAINER
--------------

    ``MAINTAINER <name>``

The ``MAINTAINER`` instruction allows you to set the *Author* field of
the generated images.

.. _dockerfile_run:

3.3 RUN
-------

RUN has 2 forms:

* ``RUN <command>`` (the command is run in a shell - ``/bin/sh -c``)
* ``RUN ["executable", "param1", "param2"]`` (*exec* form)

The ``RUN`` instruction will execute any commands in a new layer on top
of the current image and commit the results. The resulting committed image
will be used for the next step in the Dockerfile.

Layering ``RUN`` instructions and generating commits conforms to the
core concepts of Docker where commits are cheap and containers can be
created from any point in an image's history, much like source
control.

The *exec* form makes it possible to avoid shell string munging, and to ``RUN``
commands using a base image that does not contain ``/bin/sh``.

Known Issues (RUN)
..................

* :issue:`783` is about file permissions problems that can occur when
  using the AUFS file system. You might notice it during an attempt to
  ``rm`` a file, for example. The issue describes a workaround.
* :issue:`2424` Locale will not be set automatically.

.. _dockerfile_cmd:

3.4 CMD
-------

CMD has three forms:

* ``CMD ["executable","param1","param2"]`` (like an *exec*, preferred form)
* ``CMD ["param1","param2"]`` (as *default parameters to ENTRYPOINT*)
* ``CMD command param1 param2`` (as a *shell*)

There can only be one CMD in a Dockerfile. If you list more than one
CMD then only the last CMD will take effect.

**The main purpose of a CMD is to provide defaults for an executing
container.** These defaults can include an executable, or they can
omit the executable, in which case you must specify an ENTRYPOINT as
well.

When used in the shell or exec formats, the ``CMD`` instruction sets
the command to be executed when running the image.  This is
functionally equivalent to running ``docker commit -run '{"Cmd":
<command>}'`` outside the builder.

If you use the *shell* form of the CMD, then the ``<command>`` will
execute in ``/bin/sh -c``:

.. code-block:: bash

    FROM ubuntu
    CMD echo "This is a test." | wc -

If you want to **run your** ``<command>`` **without a shell** then you
must express the command as a JSON array and give the full path to the
executable. **This array form is the preferred format of CMD.** Any
additional parameters must be individually expressed as strings in the
array:

.. code-block:: bash

    FROM ubuntu
    CMD ["/usr/bin/wc","--help"]

If you would like your container to run the same executable every
time, then you should consider using ``ENTRYPOINT`` in combination
with ``CMD``. See :ref:`dockerfile_entrypoint`.

If the user specifies arguments to ``docker run`` then they will
override the default specified in CMD.

.. note::
    Don't confuse ``RUN`` with ``CMD``. ``RUN`` actually runs a
    command and commits the result; ``CMD`` does not execute anything at
    build time, but specifies the intended command for the image.

.. _dockerfile_expose:

3.5 EXPOSE
----------

    ``EXPOSE <port> [<port>...]``

The ``EXPOSE`` instruction exposes ports for use within links. This is
functionally equivalent to running ``docker commit -run '{"PortSpecs":
["<port>", "<port2>"]}'`` outside the builder. Refer to
:ref:`port_redirection` for detailed information.

.. _dockerfile_env:

3.6 ENV
-------

    ``ENV <key> <value>``

The ``ENV`` instruction sets the environment variable ``<key>`` to the
value ``<value>``. This value will be passed to all future ``RUN``
instructions. This is functionally equivalent to prefixing the command
with ``<key>=<value>``

The environment variables set using ``ENV`` will persist when a container is run
from the resulting image. You can view the values using ``docker inspect``, and change them using ``docker run --env <key>=<value>``.

.. note::
    One example where this can cause unexpected consequenses, is setting 
    ``ENV DEBIAN_FRONTEND noninteractive``.
    Which will persist when the container is run interactively; for example: 
    ``docker run -t -i image bash``

.. _dockerfile_add:

3.7 ADD
-------

    ``ADD <src> <dest>``

The ``ADD`` instruction will copy new files from <src> and add them to
the container's filesystem at path ``<dest>``.

``<src>`` must be the path to a file or directory relative to the
source directory being built (also called the *context* of the build) or
a remote file URL.

``<dest>`` is the absolute path to which the source will be copied inside the
destination container.

All new files and directories are created with mode 0755, uid and gid
0.

.. note::
   if you build using STDIN (``docker build - < somefile``), there is no build 
   context, so the Dockerfile can only contain an URL based ADD statement.

.. note::
   if your URL files are protected using authentication, you will need to use
   an ``RUN wget`` , ``RUN curl`` or other tool from within the container as
   ADD does not support authentication.

The copy obeys the following rules:

* The ``<src>`` path must be inside the *context* of the build; you cannot 
  ``ADD ../something /something``, because the first step of a 
  ``docker build`` is to send the context directory (and subdirectories) to 
  the docker daemon.
* If ``<src>`` is a URL and ``<dest>`` does not end with a trailing slash,
  then a file is downloaded from the URL and copied to ``<dest>``.
* If ``<src>`` is a URL and ``<dest>`` does end with a trailing slash,
  then the filename is inferred from the URL and the file is downloaded to
  ``<dest>/<filename>``. For instance, ``ADD http://example.com/foobar /``
  would create the file ``/foobar``. The URL must have a nontrivial path
  so that an appropriate filename can be discovered in this case
  (``http://example.com`` will not work).
* If ``<src>`` is a directory, the entire directory is copied,
  including filesystem metadata.
* If ``<src>`` is a *local* tar archive in a recognized compression
  format (identity, gzip, bzip2 or xz) then it is unpacked as a
  directory. Resources from *remote* URLs are **not** decompressed.

  When a directory is copied or unpacked, it has the same behavior as
  ``tar -x``: the result is the union of

  1. whatever existed at the destination path and
  2. the contents of the source tree,

  with conflicts resolved in favor of "2." on a file-by-file basis.

* If ``<src>`` is any other kind of file, it is copied individually
  along with its metadata. In this case, if ``<dest>`` ends with a
  trailing slash ``/``, it will be considered a directory and the
  contents of ``<src>`` will be written at ``<dest>/base(<src>)``.
* If ``<dest>`` does not end with a trailing slash, it will be
  considered a regular file and the contents of ``<src>`` will be
  written at ``<dest>``.
* If ``<dest>`` doesn't exist, it is created along with all missing
  directories in its path.

.. _dockerfile_entrypoint:

3.8 ENTRYPOINT
--------------

ENTRYPOINT has two forms:

* ``ENTRYPOINT ["executable", "param1", "param2"]`` (like an *exec*,
  preferred form)
* ``ENTRYPOINT command param1 param2`` (as a *shell*)

There can only be one ``ENTRYPOINT`` in a Dockerfile. If you have more
than one ``ENTRYPOINT``, then only the last one in the Dockerfile will
have an effect.

An ``ENTRYPOINT`` helps you to configure a container that you can run
as an executable. That is, when you specify an ``ENTRYPOINT``, then
the whole container runs as if it was just that executable.

The ``ENTRYPOINT`` instruction adds an entry command that will **not**
be overwritten when arguments are passed to ``docker run``, unlike the
behavior of ``CMD``.  This allows arguments to be passed to the
entrypoint.  i.e. ``docker run <image> -d`` will pass the "-d"
argument to the ENTRYPOINT.

You can specify parameters either in the ENTRYPOINT JSON array (as in
"like an exec" above), or by using a CMD statement. Parameters in the
ENTRYPOINT will not be overridden by the ``docker run`` arguments, but
parameters specified via CMD will be overridden by ``docker run``
arguments.

Like a ``CMD``, you can specify a plain string for the ENTRYPOINT and
it will execute in ``/bin/sh -c``:

.. code-block:: bash

    FROM ubuntu
    ENTRYPOINT wc -l -

For example, that Dockerfile's image will *always* take stdin as input
("-") and print the number of lines ("-l"). If you wanted to make
this optional but default, you could use a CMD:

.. code-block:: bash

    FROM ubuntu
    CMD ["-l", "-"]
    ENTRYPOINT ["/usr/bin/wc"]

.. _dockerfile_volume:

3.9 VOLUME
----------

    ``VOLUME ["/data"]``

The ``VOLUME`` instruction will create a mount point with the specified name and mark it 
as holding externally mounted volumes from native host or other containers. For more information/examples 
and mounting instructions via docker client, refer to :ref:`volume_def` documentation. 

.. _dockerfile_user:

3.10 USER
---------

    ``USER daemon``

The ``USER`` instruction sets the username or UID to use when running
the image.

.. _dockerfile_workdir:

3.11 WORKDIR
------------

    ``WORKDIR /path/to/workdir``

The ``WORKDIR`` instruction sets the working directory for the ``RUN``, ``CMD`` and
``ENTRYPOINT``  Dockerfile commands that follow it.

It can be used multiple times in the one Dockerfile.

3.11 ONBUILD
------------

    ``ONBUILD [INSTRUCTION]``

The ``ONBUILD`` instruction adds to the image a "trigger" instruction to be
executed at a later time, when the image is used as the base for another build.
The trigger will be executed in the context of the downstream build, as if it
had been inserted immediately after the *FROM* instruction in the downstream
Dockerfile.

Any build instruction can be registered as a trigger.

This is useful if you are building an image which will be used as a base to build
other images, for example an application build environment or a daemon which may be
customized with user-specific configuration.

For example, if your image is a reusable python application builder, it will require
application source code to be added in a particular directory, and it might require
a build script to be called *after* that. You can't just call *ADD* and *RUN* now,
because you don't yet have access to the application source code, and it will be
different for each application build. You could simply provide application developers
with a boilerplate Dockerfile to copy-paste into their application, but that is
inefficient, error-prone and difficult to update because it mixes with
application-specific code.

The solution is to use *ONBUILD* to register in advance instructions to run later,
during the next build stage.

Here's how it works:

1. When it encounters an *ONBUILD* instruction, the builder adds a trigger to
   the metadata of the image being built.
   The instruction does not otherwise affect the current build.

2. At the end of the build, a list of all triggers is stored in the image manifest,
   under the key *OnBuild*. They can be inspected with *docker inspect*.

3. Later the image may be used as a base for a new build, using the *FROM* instruction.
   As part of processing the *FROM* instruction, the downstream builder looks for *ONBUILD*
   triggers, and executes them in the same order they were registered. If any of the
   triggers fail, the *FROM* instruction is aborted which in turn causes the build
   to fail. If all triggers succeed, the FROM instruction completes and the build
   continues as usual.

4. Triggers are cleared from the final image after being executed. In other words
   they are not inherited by "grand-children" builds.

For example you might add something like this:

.. code-block:: bash

    [...]
    ONBUILD ADD . /app/src
    ONBUILD RUN /usr/local/bin/python-build --dir /app/src
    [...]

.. warning:: Chaining ONBUILD instructions using `ONBUILD ONBUILD` isn't allowed.
.. warning:: ONBUILD may not trigger FROM or MAINTAINER instructions.

.. _dockerfile_examples:

4. Dockerfile Examples
======================

.. code-block:: bash

    # Nginx
    #
    # VERSION               0.0.1

    FROM      ubuntu
    MAINTAINER Guillaume J. Charmes <guillaume@dotcloud.com>

    # make sure the package repository is up to date
    RUN echo "deb http://archive.ubuntu.com/ubuntu precise main universe" > /etc/apt/sources.list
    RUN apt-get update

    RUN apt-get install -y inotify-tools nginx apache2 openssh-server

.. code-block:: bash

    # Firefox over VNC
    #
    # VERSION               0.3

    FROM ubuntu
    # make sure the package repository is up to date
    RUN echo "deb http://archive.ubuntu.com/ubuntu precise main universe" > /etc/apt/sources.list
    RUN apt-get update

    # Install vnc, xvfb in order to create a 'fake' display and firefox
    RUN apt-get install -y x11vnc xvfb firefox
    RUN mkdir /.vnc
    # Setup a password
    RUN x11vnc -storepasswd 1234 ~/.vnc/passwd
    # Autostart firefox (might not be the best way, but it does the trick)
    RUN bash -c 'echo "firefox" >> /.bashrc'

    EXPOSE 5900
    CMD    ["x11vnc", "-forever", "-usepw", "-create"]

.. code-block:: bash

    # Multiple images example
    #
    # VERSION               0.1

    FROM ubuntu
    RUN echo foo > bar
    # Will output something like ===> 907ad6c2736f

    FROM ubuntu
    RUN echo moo > oink
    # Will output something like ===> 695d7793cbe4

    # You'll now have two images, 907ad6c2736f with /bar, and 695d7793cbe4 with
    # /oink.