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A Dockerfile-replacement DSL for building complex images


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~> 0.34
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 Project Readme


WARNING: Work in progress. Although this software has test coverage, it is considered unstable. Refer to LICENSE for licensing information.

See section on Compatibility for a list of supported Docker versions for every Drydock version. See the VERSION file for the version of Drydock to which this source code is associated.

Automated Build Status Code Climate

A ruby DSL to build your own docker images. Images are built based on instructions contained in your project's Drydockfile.

Why not Dockerfile?

Dockerfiles are great to start out with, but:

  1. They are static, which isn't necessarily a bad thing. I'm not opposed to a declarative approach to building images, but sometimes it may be limiting.
  2. They are less hackable, because docker build doesn't support plugins to expand its capabilities.
  3. More complicated build pipelines are hard to implement, or perhaps even impossible. For instance, being able to build multiple images, then combine them at the end, would be a nice option. Imagine building your ruby gem dependencies and node.js dependencies separately, before combining both images into your application's final image.
  4. Caching rules are fairly limiting. For instance, when your Gemfile changes, it would be nice to import a configurably-older image, import the new Gemfile, and re-run the build. On the other hand, it would be important to be able to limit the age of the cache.

Why Drydock?

Drydock interfaces directly with the Docker Remote API via the docker-api gem. It's not for every use case, but it provides great control over what and how an image is built.

Drydock supports plugins, either provided through ruby gems or ruby files included in your project being built by Drydock.

Drydockfiles are written in ruby.


Either (a) gem install dry-dock, or (b) add "dry-dock" to your project's Gemfile, and run bundle.

Sorry, but the gem name drydock was already taken by a defunct gem, and I'm too lazy to contact them; the binary and name of the project, however, are both drydock.

In your project's root directory, you'll want to create a Drydockfile containing drydock functions. When you're ready, from your project's directory, build an image using:

$ bundle exec drydock

or drydock directly if you're not using bundler.

Alternatively, point drydock to a directory containing the Drydockfile, or to any file to treat it as the Drydockfile, e.g.:

$ drydock ~/source/miniproject # project directory expects a file named Drydockfile
$ drydock ~/source/miniproject/drydock-definition.rb # expects a drydock-definition.rb

Example Drydockfiles may be seen in the examples/ directory of the source repo.

Drydockfile Syntax

As previously mentioned, Drydockfiles are ruby. The contents of Drydockfile are evaluated in the context of an instance of {Drydock::Project}; you can refer to the documentation for it for more in-depth information on each instruction.

Because Drydockfiles are ruby, most constructs should work as-is: you can declare constants and refer to them later; call Kernel#abort to exit the program and stop the build; and write plugins to be called from within the Drydockfile.

It would help if you understand ruby and Dockerfiles before jumping in.

All instructions are evaluated in the order that they are seen; syntax errors or any logical errors might not be caught until execution arrives at that point.

For a complete and updated list of Drydockfile instructions, see the public API methods of the {Drydock::Project} class or head to the automatically-generated ruby docs.


If you plan on hacking or contributing to drydock, fork the project, create a new branch, make your changes, commit, and open a pull request.

After cloning your repo, bundle should take care of it.


$ bundle
$ # increment VERSION file
$ bundle exec rake gemspec build
$ # upload .gem file to


  1. Customizable caching subsystem with pluggable caching strategies.
  2. Squashing layers together, with cache support.
  3. Unarchiving a file directly into a container.
  4. Proper ONBUILD implementation and expanded support for hooks.
  5. Drydock instructions corresponding to LABEL, VOLUME, USER, and WORKDIR Docker instructions.


The following version combinations are officially tested and supported:

Docker Versions Drydock Versions
v1.8.0 v0.1.0 onwards
v1.9.0 v0.2.0 onwards

Docker v1.7 or earlier is not officially supported, but most functionality should work, with the exception of:

  • The copy command, which may fail when unpacking into the root / of the container.
  • The import command, which requires the /containers/(id)/archive. Earlier versions of the Docker Remote API implemented /containers/(id)/copy; if you'd like to add graceful fallback using the aforementioned, contributions are welcome.