/etc

tags: docker

There are a few different strategies for getting your Git source code into a Docker build. Many of these have different ways of interacting with Docker’s caching mechanisms, and may be more or less appropriately suited to your project and how you intend to use Docker. Perhaps surprisingly, I haven’t been able to locate an overview of these strategies collected in one place, and it’s not covered in the Dockerfile best practices guide.

Here are the strategies I’ve come across so far:

RUN git clone

If you’re like me, this is the approach that first springs to mind when you see the commands available to you in a Dockerfile. The trouble with this is that it can interact in several unintuitive ways with Docker’s build caching mechanisms. For example, if you make an update to your git repository, and then re-run the docker build which has a RUN git clone command, you may or may not get the new commit(s) depending on if the preceding Dockerfile commands have invalidated the cache.

One way to get around this is to use docker build --no-cache, but then if there are any time-intensive commands preceding the clone they’ll have to run again too.

Another issue is that you (or someone you’ve distributed your Dockerfile to) may unexpectedly come back to a broken build later on when the upstream git repository updates.

A two-birds-one-stone approach to this while still using RUN git clone is to put it on one line1 with a specific revision checkout, e.g.:

RUN git clone https://github.com/example/example.git && cd example && git checkout 0123abcdef

Then updating the revision to check out in the Dockerfile will invalidate the cache at that line and cause the clone/checkout to run.

One possible drawback to this approach in general is that you have to have git installed in your container.

RUN curl or ADD a tag/commit tarball URL

This avoids having to have git installed in your container environment, and can benefit from being explicit about when the cache will break (i.e. if the tag/revision is part of the URL, that URL change will bust the cache). Note that if you use the Dockerfile ADD command to copy from a remote URL, the file will be downloaded every time you run the build, and the HTTP Last-Modified header will also be used to invalidate the cache.

You can see this approach used in the golang Dockerfile.

Git submodules inside Dockerfile repository

If you keep your Dockerfile and Docker build in a separate repository from your source code, or your Docker build requires multiple source repositories, using git submodules (or git subtrees) in this repository may be a valid way to get your source repos into your build context. This avoids some concerns with Docker caching and upstream updating, as you lock the upstream revision in your submodule/subtree specification. Updating them will break your Docker cache as it changes the build context.

Note that this only gets the files into your Docker build context, you still need to use ADD commands in your Dockerfile to copy those paths to where you expect them in the container.

I use this approach in my Docker for Latin OCR training repository.

Dockerfile inside git repository

Here, you just have your Dockerfile in the same git repository alongside the code you want to build/test/deploy, so it automatically gets sent as part of the build context, so you can e.g. ADD . /project to copy the context into the container. The advantage to this is that you can test changes without having to potentially commit/push them to get them into a test docker build; the disadvantage is that every time you modify any files in your working directory it will invalidate the cache at the ADD command. Sending the build context for a large source/data directory can also be time-consuming. So if you use this approach, you may also want to make judicious use of the .dockerignore file, including doing things like ignoring everything in your .gitignore and possibly the .git directory itself. You may also want to ignore the Dockerfile in your .dockerignore, as you are unlikely to be using the Dockerfile inside the container, and otherwise it will invalidate the cache at the ADD line every time you change your Dockerfile.

Volume mapping

If you’re using Docker to set up a dev/test environment that you want to share among a wide variety of source repos on your host machine, mounting a host directory as a data volume may be a viable strategy. This gives you the ability to specify which directories you want to include at docker run-time, and avoids concerns about docker build caching, but none of this will be shared among other users of your Dockerfile or container image.

Footnotes

  1. The reason to put this on one line is the same reason you shouldn’t put RUN apt-get update on a single line. Consider instead the form:

      RUN git clone https://github.com/example/example.git
      RUN cd example && git checkout 0123abcdef
    

    Here, if you update the revision, the clone command will still use the cache while the checkout won’t, and you may try to check out a revision which isn’t in the cache.