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Docker `with_build_env.sh` Script

Seán Kelleher
・9 min read

In Docker for the Build Process, I introduced the idea of extracting the build phase of a project into a dedicated build_img.sh script, which uses the project's Dockerised build environment to build the "run" image for the project, allowing a project to be built on a host environment without installing any extra tooling other than Docker. In this post I'll expand on this idea and show how we can use this script as a basis for a new, reusable script that allows us to easily work with the build environment of our project.

Basic Usage

The idea here is to create a with_build_env.sh script which, at its most basic, will take a command and run it in the build environment, with the local project directory also mounted in this build environment. This means that we could, for example, run bash scripts/with_build_env.sh make without even installing make locally, but have all the artefacts output to our local project directory.

Headless usage

The first way this script can be run is the "headless" way, which is the approach that will be primarily used in the build pipeline. This runs a command in the build environment:

bash scripts/with_build_env.sh make posts
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More complicated commands involving Bash operators can also be performed using the likes of bash -c:

bash scripts/with_build_env.sh bash -c 'make check && make posts'
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Interactive usage

The second way this script can be run is the "interactive" way, which will only be used locally in general. This typically involves running an interactive shell in the build environment. This will allow you to run build environment tools on your project, even if they're not installed on your local environment.

This approach will usually be performed using sh/bash as the command, and using a flag to indicate that the current command should be run interactively:

bash scripts/with_build_env.sh --dev bash
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This launches a Bash process in the build environment. This allows you to work within your local directory but use the tools from your build environment.

The reason for using a flag to distinguish interactive use from headless use is that build pipelines generally don't provide a certain mechanism (a TTY) for interactive use, so attempting to run a command in interactive mode in the build pipeline will fail.

Basic Implementation

The following is a basic with_build_env.sh script based on the ideas presented in Docker for the Build Process:

org='ezanmoto'
proj='hello'
build_img="$org/$proj.build"

bash scripts/docker_rbuild.sh \
    "$build_img" \
    "latest" \
    --file='build.Dockerfile' \
    .

workdir='/go/src/github.com/ezanmoto/hello'

docker run \
    --rm \
    --mount="type=bind,src=$(pwd),dst=$workdir" \
    --workdir="$workdir" \
    "$build_img:latest" \
    "$@"
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This just performs two actions: it rebuilds the image for the build environment, if necessary, and then runs the provided command in the build environment. Rebuilding the image isn't necessary if you're using a remote image, but this step is useful for keeping your image up-to-date if your project defines its own build environment, as it's common for the requirements of projects to grow beyond base images quite quickly.

Using this script to build the current project can then be as straightforward as bash scripts/with_build_env.sh make. However, there are a number of drawbacks to the script as it currently is, such as the fact that the build runs as root and dependencies aren't cached. The following sections show optional, incremental steps that can be used to improve this basic implementation.

One additional note about with_build_env.sh is that, while the general idea and approach of each instance of the script is the same, each specific instance of the script may vary. This is because the utility of this script will generally change from project to project and language to language. For example, when a with_build_env.sh script wants to keep the build cache between Docker runs, the specifics of where the cache lives is handled directly by the with_build_env.sh script, and this changes depending on the language and build tooling being used.

Layers

The basic with_build_env.sh script presented above gives a lot of the benefits touted in Docker for the Build Process right out of the box. However, you are likely to encounter different issues depending on exactly how you'll be using the script. For example, the first issue that is likely to be encountered when working with this script is to try and run it in a build pipeline, which will probably result in Docker failing with an error output of the input device is not a TTY. Another problem is that the issued command is run by root by default which, while not necessarily a problem in and of itself, can cause a little friction when files created in the build environment are owned by root in the host directory. This section outlines the most common and problematic issues that may be encountered, along with possible resolutions.

Interactive use

The first issue that is often encountered when working with the with_build_env.sh script is the fact that Docker will need the --interactive and --tty flags when running locally, but will need them to be removed when running in the build pipeline. For this I generally introduce some basic argument parsing to the script to allow it to either be run in "dev" (interactive) mode, or not:

+docker_flags=''
+case "$1" in
+    --dev)
+        docker_flags="$docker_flags --interactive --tty --publish=$2"
+        shift 2
+        ;;
+esac
+
 org='ezanmoto'
 proj='hello'
 build_img="$org/$proj.build"

 bash scripts/docker_rbuild.sh \
     "$build_img" \
     "latest" \
     --file='build.Dockerfile' \
     .

 workdir='/go/src/github.com/ezanmoto/hello'

 docker run \
+    $docker_flags \
     --rm \
     --mount="type=bind,src=$(pwd),dst=$workdir" \
     --workdir="$workdir" \
     "$build_img:latest" \
     "$@"
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Passing --interactive and --tty is sufficient for enabling this functionality, but I have a convention of also having the --dev option take a port forwarding argument, which is used to expose a port from the container. This is because I often find myself making use of ports to achieve some level of communication between the host and the container, such as running the main service I'm working on in the build environment and accessing it from the host. This addition does mean that the command for launching an interactive Bash shell has to be modified slightly, but it also means that we avoid having to restart the session in order to expose the container to the host:

bash scripts/with_build_env.sh --dev 3000:3000 bash
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User mapping

This isn't as much of an issue in the build pipeline, but when using the with_build_env.sh script as presented above, one issue is that the default user in the build environment is root. This is fine in a functional sense - you'll generally be able to build the project without issue and won't run into ownership problems. However, it quickly becomes very tedious from a usability perspective - any files that are created in the container are owned by root, requiring sudo to remove them locally, and accidentally performing git operations can result in pain down the line as some of the files in your .git directory can have their ownership altered.

As a more usable solution I usually pass --user="$(id --user):$(id --group)" to the docker run command. This means that we're now running as our host user when using the build environment, so any files we create will have the desired ownership:

 docker run \
     $docker_flags \
     --rm \
+    --user="$(id --user):$(id --group)" \
     --mount="type=bind,src=$(pwd),dst=$workdir" \
     --workdir="$workdir" \
     "$build_img:latest" \
     "$@"
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User mapping caveats

One issue with mapping the user as presented is that, while we're using the correct user and group IDs in the container for local development, this user doesn't actually exist within the build environment. This means that any tools that rely on the user's $HOME, including many Git and SSH-based commands, simply won't work. Such commands will either need to be run outside the build environment (such as git commit), or else the build environment will need to be set up with a functional user to carry out specific commands with sudo. It's not always necessary for the user to exist in the container, but if it is then that user can be created in the build environment with the desired user ID and group ID.

Caching

A convention of with_build_env.sh is to --rm containers after each use, which is useful to avoid accidentally depending on ephemeral aspects of the build environment. However, this means that any project dependencies stored outside the project directory must be re-downloaded with each launch of the build environment.

The solution to this is to cache the downloaded files. This is a big area where the script will change based on the programming language and tooling being used.

The first step is to create an area to persist the cached directories. For this I create named volumes with open permissions:

tmp_cache="$org.$proj.tmp_cache"
pkg_cache="$org.$proj.pkg_cache"
tmp_cache_dir='/tmp/cache'
pkg_cache_dir='/go/pkg'

docker run \
    --rm \
    --mount="type=volume,src=$tmp_cache,dst=$tmp_cache_dir" \
    --mount="type=volume,src=$pkg_cache,dst=$pkg_cache_dir" \
    "$build_img:latest" \
    chmod \
        0777 \
        "$tmp_cache_dir" \
        "$pkg_cache_dir"
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The specific image being used here isn't important (it just needs to have chmod present) but we use the build environment for this for simplicity. We give the directory open permissions because volumes are owned by root when created by docker, and we want to allow any user to be able to download dependencies and run builds in the build environment.

It's also useful to prefix the name of the volume with the name of the project (ezanmoto.hello., in this example) to help isolate builds across project boundaries. See the "Caveats" section, below, for more details.

The last piece of the puzzle then is to mount the volume when using the build environment, and to let the build tools that we're using know about this:

 docker run \
     $docker_flags \
     --rm \
+    --env=XDG_CACHE_HOME="$tmp_cache_dir" \
+    --mount="type=volume,src=$tmp_cache,dst=$tmp_cache_dir" \
+    --mount="type=volume,src=$pkg_cache,dst=$pkg_cache_dir" \
     --user="$(id --user):$(id --group)" \
     --mount="type=bind,src=$(pwd),dst=$workdir" \
     --workdir="$workdir" \
     "$build_img:latest" \
     "$@"
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We can see that the Go build tools are told about the cache directory through the use of the XDG_CACHE_HOME environment variable.

This setup allows the persistence of project dependencies across docker runs without baking them into the image or storing them locally. The cache can also be cleared by running docker volume rm ezanmoto.hello.tmp_cache ezanmoto.hello.pkg_cache, and the cache area will be automatically remade the next time with_build_env.sh runs.

Rust

The following shows how a similar caching mechanism could be implemented for Rust. This snippet is taken from another project of mine:

docker run \
    --rm \
    --mount='type=volume,src=dpnd_cargo_cache,dst=/cargo' \
    "$build_img:latest" \
    chmod 0777 /cargo

docker run \
    --interactive \
    --tty \
    --rm \
    --mount='type=volume,src=dpnd_cargo_cache,dst=/cargo' \
    --env='CARGO_HOME=/cargo' \
    --user="$(id -u):$(id -g)" \
    --mount="type=bind,src=$(pwd),dst=/app" \
    --workdir='/app' \
    "$build_img:latest" \
    "$@"
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It can be seen that Rust uses the CARGO_HOME environment variable to locate the cache.

Caveats of cache volumes

It should generally be fine to use the same cache across projects, but some tools can encounter difficulties when the same caching volume is shared between projects (I've experienced this when using Go in particular, where I've encountered issues with the checksum database).

The simplest solution in this scenario is to prefixing the volume name with the name of the project in order to isolate volumes per project. However, do note that this issue can still arise even in a single project - for example, when changing between branches with different lockfiles. In this scenario it's helpful to remove the cache volume and allow with_build_env.sh to recreate it:

docker volume rm ezanmoto.hello.tmp_cache ezanmoto.hello.pkg_cache
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Benefits

An immediate benefit of with_build_env.sh is that, assuming direct Docker access is available to us in the build pipeline, we can use the same commands locally those used in the build pipeline. This makes our pipeline easier to set up, but also means that we can run our local code in the build environment before we commit to help ensure that code that builds locally will also build in the build pipeline.

Some other benefits:

  • We have a simpler mechanism to open a shell in the build environment, which can be used for building, testing and debugging.

  • We can more easily work with a project without having to manually install its dependencies and required tools, which helps us avoid issues when trying to run the project for the first time. This is particularly helpful for onboarding new developers.

  • Many of the benefits outlined in Docker for the Build Process also apply. In particular, it now becomes much easier to start the interactive build environment.

Conclusion

The with_build_env.sh script allows us to abstract the process of running commands inside the build environment, making it easier for us to build, run and debug code in the replicated build environment. This enables greater parity with the build pipeline, and further helps to avoid the classic "it works on my machine" issue.


This article was originally published on seankelleher.ie.

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