254 lines
11 KiB
Markdown
254 lines
11 KiB
Markdown
Docker: the Linux container engine
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==================================
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Docker is an open source project to pack, ship and run any application
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as a lightweight container.
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Docker containers are both *hardware-agnostic* and *platform-agnostic*.
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This means they can run anywhere, from your laptop to the largest
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EC2 compute instance and everything in between - and they don't require
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you to use a particular language, framework or packaging system. That
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makes them great building blocks for deploying and scaling web apps,
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databases, and backend services without depending on a particular stack
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or provider.
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Docker began as an open-source implementation of the deployment engine which
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powers [dotCloud](https://dotcloud.com), a popular Platform-as-a-Service.
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It benefits directly from the experience accumulated over several years
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of large-scale operation and support of hundreds of thousands of
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applications and databases.
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## Security Disclosure
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Security is very important to us. If you have any issue regarding security,
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please disclose the information responsibly by sending an email to
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security@docker.com and not by creating a github issue.
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## Better than VMs
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A common method for distributing applications and sandboxing their
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execution is to use virtual machines, or VMs. Typical VM formats are
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VMWare's vmdk, Oracle Virtualbox's vdi, and Amazon EC2's ami. In theory
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these formats should allow every developer to automatically package
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their application into a "machine" for easy distribution and deployment.
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In practice, that almost never happens, for a few reasons:
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* *Size*: VMs are very large which makes them impractical to store
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and transfer.
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* *Performance*: running VMs consumes significant CPU and memory,
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which makes them impractical in many scenarios, for example local
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development of multi-tier applications, and large-scale deployment
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of cpu and memory-intensive applications on large numbers of
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machines.
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* *Portability*: competing VM environments don't play well with each
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other. Although conversion tools do exist, they are limited and
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add even more overhead.
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* *Hardware-centric*: VMs were designed with machine operators in
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mind, not software developers. As a result, they offer very
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limited tooling for what developers need most: building, testing
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and running their software. For example, VMs offer no facilities
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for application versioning, monitoring, configuration, logging or
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service discovery.
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By contrast, Docker relies on a different sandboxing method known as
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*containerization*. Unlike traditional virtualization, containerization
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takes place at the kernel level. Most modern operating system kernels
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now support the primitives necessary for containerization, including
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Linux with [openvz](https://openvz.org),
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[vserver](http://linux-vserver.org) and more recently
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[lxc](http://lxc.sourceforge.net), Solaris with
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[zones](https://docs.oracle.com/cd/E26502_01/html/E29024/preface-1.html#scrolltoc),
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and FreeBSD with
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[Jails](https://www.freebsd.org/doc/handbook/jails.html).
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Docker builds on top of these low-level primitives to offer developers a
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portable format and runtime environment that solves all four problems.
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Docker containers are small (and their transfer can be optimized with
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layers), they have basically zero memory and cpu overhead, they are
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completely portable, and are designed from the ground up with an
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application-centric design.
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Perhaps best of all, because Docker operates at the OS level, it can still be
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run inside a VM!
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## Plays well with others
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Docker does not require you to buy into a particular programming
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language, framework, packaging system, or configuration language.
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Is your application a Unix process? Does it use files, tcp connections,
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environment variables, standard Unix streams and command-line arguments
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as inputs and outputs? Then Docker can run it.
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Can your application's build be expressed as a sequence of such
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commands? Then Docker can build it.
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## Escape dependency hell
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A common problem for developers is the difficulty of managing all
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their application's dependencies in a simple and automated way.
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This is usually difficult for several reasons:
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* *Cross-platform dependencies*. Modern applications often depend on
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a combination of system libraries and binaries, language-specific
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packages, framework-specific modules, internal components
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developed for another project, etc. These dependencies live in
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different "worlds" and require different tools - these tools
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typically don't work well with each other, requiring awkward
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custom integrations.
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* *Conflicting dependencies*. Different applications may depend on
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different versions of the same dependency. Packaging tools handle
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these situations with various degrees of ease - but they all
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handle them in different and incompatible ways, which again forces
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the developer to do extra work.
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* *Custom dependencies*. A developer may need to prepare a custom
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version of their application's dependency. Some packaging systems
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can handle custom versions of a dependency, others can't - and all
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of them handle it differently.
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Docker solves the problem of dependency hell by giving the developer a simple
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way to express *all* their application's dependencies in one place, while
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streamlining the process of assembling them. If this makes you think of
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[XKCD 927](https://xkcd.com/927/), don't worry. Docker doesn't
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*replace* your favorite packaging systems. It simply orchestrates
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their use in a simple and repeatable way. How does it do that? With
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layers.
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Docker defines a build as running a sequence of Unix commands, one
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after the other, in the same container. Build commands modify the
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contents of the container (usually by installing new files on the
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filesystem), the next command modifies it some more, etc. Since each
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build command inherits the result of the previous commands, the
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*order* in which the commands are executed expresses *dependencies*.
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Here's a typical Docker build process:
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```bash
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FROM ubuntu:12.04
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RUN apt-get update && apt-get install -y python python-pip curl
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RUN curl -sSL https://github.com/shykes/helloflask/archive/master.tar.gz | tar -xzv
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RUN cd helloflask-master && pip install -r requirements.txt
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```
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Note that Docker doesn't care *how* dependencies are built - as long
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as they can be built by running a Unix command in a container.
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Getting started
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===============
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Docker can be installed on your local machine as well as servers - both
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bare metal and virtualized. It is available as a binary on most modern
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Linux systems, or as a VM on Windows, Mac and other systems.
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We also offer an [interactive tutorial](https://www.docker.com/tryit/)
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for quickly learning the basics of using Docker.
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For up-to-date install instructions, see the [Docs](https://docs.docker.com).
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Usage examples
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==============
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Docker can be used to run short-lived commands, long-running daemons
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(app servers, databases, etc.), interactive shell sessions, etc.
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You can find a [list of real-world
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examples](https://docs.docker.com/examples/) in the
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documentation.
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Under the hood
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--------------
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Under the hood, Docker is built on the following components:
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* The
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[cgroup](http://blog.dotcloud.com/kernel-secrets-from-the-paas-garage-part-24-c)
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and
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[namespacing](http://blog.dotcloud.com/under-the-hood-linux-kernels-on-dotcloud-part)
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capabilities of the Linux kernel
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* The [Go](https://golang.org) programming language
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* The [Docker Image Specification](https://github.com/docker/docker/blob/master/image/spec/v1.md)
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* The [Libcontainer Specification](https://github.com/docker/libcontainer/blob/master/SPEC.md)
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Contributing to Docker
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======================
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[](https://godoc.org/github.com/docker/docker)
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[](https://jenkins.dockerproject.com/job/Docker%20Master/)
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Want to hack on Docker? Awesome! We have [instructions to help you get
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started contributing code or documentation.](https://docs.docker.com/project/who-written-for/).
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These instructions are probably not perfect, please let us know if anything
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feels wrong or incomplete. Better yet, submit a PR and improve them yourself.
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Getting the development builds
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==============================
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Want to run Docker from a master build? You can download
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master builds at [master.dockerproject.com](https://master.dockerproject.com).
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They are updated with each commit merged into the master branch.
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Don't know how to use that super cool new feature in the master build? Check
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out the master docs at
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[docs.master.dockerproject.com](http://docs.master.dockerproject.com).
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How the project is run
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======================
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Docker is a very, very active project. If you want to learn more about how it is run,
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or want to get more involved, the best place to start is [the project directory](https://github.com/docker/docker/tree/master/project).
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We are always open to suggestions on process improvements, and are always looking for more maintainers.
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### Legal
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*Brought to you courtesy of our legal counsel. For more context,
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please see the [NOTICE](https://github.com/docker/docker/blob/master/NOTICE) document in this repo.*
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Use and transfer of Docker may be subject to certain restrictions by the
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United States and other governments.
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It is your responsibility to ensure that your use and/or transfer does not
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violate applicable laws.
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For more information, please see https://www.bis.doc.gov
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Licensing
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=========
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Docker is licensed under the Apache License, Version 2.0. See
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[LICENSE](https://github.com/docker/docker/blob/master/LICENSE) for the full
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license text.
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Other Docker Related Projects
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=============================
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There are a number of projects under development that are based on Docker's
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core technology. These projects expand the tooling built around the
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Docker platform to broaden its application and utility.
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* [Docker Registry](https://github.com/docker/distribution): Registry
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server for Docker (hosting/delivery of repositories and images)
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* [Docker Machine](https://github.com/docker/machine): Machine management
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for a container-centric world
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* [Docker Swarm](https://github.com/docker/swarm): A Docker-native clustering
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system
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* [Docker Compose](https://github.com/docker/compose) (formerly Fig):
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Define and run multi-container apps
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* [Kitematic](https://github.com/kitematic/kitematic): The easiest way to use
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Docker on a Mac
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If you know of another project underway that should be listed here, please help
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us keep this list up-to-date by submitting a PR.
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Mailing Lists
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=============
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* [Google group for Docker Users](https://groups.google.com/forum/#!forum/docker-user)
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* [Google group for Docker development discussions](https://groups.google.com/forum/#!forum/docker-dev)
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