DoGet

Dockerfiles can only inherit from one source. What if you want to want to your favorite application on your own base image? Then you're down to copy&pasting Dockerfile code all over the place.

DoGet solves this. Think of DoGet as "compiler-assisted copy&paste". Here's an example:

FROM corporate.example.com/images/debian:8        # Our own base image

PROVIDES debian:jessie                            # ...which is basically Debian Jessie

USE github.com/docker-library/php/7.0             # On top of that, use official PHP 7.0

# <Insert app here>

DoGet extends Dockerfile syntax with PROVIDES and USE instructions. The USE instruction downloads the Dockerfile from the specified location and includes its contents (as if it had been copy&pasted). The FROM instruction in included files is checked for compatibility. The PROVIDES instruction serves the purpose of satisfying the compatibility check.

To use the tool, copy the above into a file called Dockerfile.in and type:

$ doget build -t [tag] .

This will resolve traits, downloading if necessary, and pass on the created Dockerfile (and any command line options) to docker build.

Installing

You will need Go >= 1.6 installed. Then, type:

$ go get gopkg.in/yaml.v2
$ go get github.com/tueftler/doget
$ go build -o ~/bin/doget github.com/tueftler/doget

Using traits

Traits are just regular Dockerfiles stored somewhere on GitHub or BitBucket. You can reference them by using a domain/vendor/repo[/dir][:version] syntax.

Example: github.com/docker-library/php/7.0 will use the Dockerfile in docker-library/php >> 7.0

By default, this will check out the master branch. To reference a version, you can either use commit SHAs, branch names or tags and append them, e.g. github.com/thekid/traits/xp:v1.0.0.

Authoring traits

As said, traits are nothing special. Just commit and push them to make them available to the public. However, if you're creating Dockerfiles specifically designed for reuse, here are some things to keep in mind:

• Always add a FROM instruction to express what your Dockerfile extends from.
• If your traits provides an official base image, use PROVIDES and add its name.
• You can use USE to declare transitive dependencies. If you do so, you should reference a specific version, otherwise you risk problems at a later point.
• Think twice about adding an ENTRYPOINT or CMD, people will typically want to do this themselves.
• Test it using a continuous integration system like Travis CI
• Use semantic versioning and keep a changelog

Caching

DoGet caches downloaded traits inside the working directory. Their contents are stored zipped in a file called doget_modules.zip. To force a fresh download, simply remove this file.

You can check the file in to your SCM - this way, you can create repeatable builds even if the remote locations should not be reachable at build time.

Contributing

To contribute to DoGet, use the way - fork, hack, and submit a pull request! If you're unsure where to start, look out for issues labeled with help wanted.

Enjoy!