Build your own Earthly CI Image

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Introduction
This guide is intended to help you create your own Docker image with Earthly inside it for your containerized CI workflows.
Getting Started
There are two ways to build a containerized CI image with Earthly:
Extending the earthly/earthly image with an external runner/agent
Adding Earthly to an existing image
This guide will cover both approaches to constructing your image.
Extending The earthly/earthly Image
This is the recommended approach when adopting Earthly into your containerized CI. Start by basing your custom image on ours:
FROM earthly/earthly:v0.6.21
RUN ... # Add your agent, certificates, tools...
When extending our image, be sure to pin to a specific version to avoid accidental future breakage as earthly evolves.
The earthly/earthly image is Alpine Linux based. To add tools to the image, you can use apk:
apk add --no-cache my-cool-tool
If you are adding a tool from outside the Alpine Linux repositories, test it to ensure it is compatible. Alpine uses musl, which can create incompatibilities with some software.
Also, you should embed any configuration that your Earthly image might need (to avoid having it in your build scripts, or mounted from a host somewhere). You can do this in-line with the earthly config command.
Adding Earthly To An Existing Image
This section will cover adding Earthly to an existing image when:
Docker-In-Docker is configured for the base image
Earthly will be connecting to a remote earthly/buildkitd instance
While it is possible to configure a locally-ran earthly/buildkitd instance within an image (it's how earthly/earthly works), the steps and tweaks are beyond the scope of this guide.
Docker-In-Docker
In this setup, Earthly will be allowed to manage an instance of its earthly/buildkitd daemon over a live Docker socket.
To enable this, simply follow the installation instructions within your Dockerfile/Earthfile as you would on any other host. An example of installing this can be found below.
RUN wget https://github.com/earthly/earthly/releases/download/v0.6.21/earthly-linux-amd64 -O /usr/local/bin/earthly && \
    chmod +x /usr/local/bin/earthly && \
    /usr/local/bin/earthly bootstrap
As with the Docker containers, be sure to pin the version in the download URL to avoid any accidental future breakage. Assuming Docker is also installed and available, you should be able to invoke Earthly without any additional configuration.
Remote Daemon
When connecting to a remote daemon, follow the Docker-In-Docker installation instructions above to get the binary. Then you'll need to issue a few earthly config commands to ensure the container is set up to automatically use the remote daemon. It might look something like this:
RUN earthly config global.buildkit_host buildkit_host: 'tcp://myhost:8372'
For more details on using a remote BuildKit daemon, see our guide.
cgroups v2 Considerations
When cgroups v2 is detected by the earthly/earthly image's default entrypoint, it moves it's process under an isolated cgroup. If a different entrypoint is used (i.e. a custom user supplied script), the root process must be moved into a separate cgroup, for example:
if [ -f "/sys/fs/cgroup/cgroup.controllers" ]; then
    echo "detected cgroups v2; moving pid $ to subgroup"
​
    # move the process under a new cgroup to prevent buildkitd/entrypoint.sh
    # from getting a "sh: write error: Resource busy" error while enabling controllers
    # via echo +pids > /sys/fs/cgroup/cgroup.subtree_control
    mkdir -p /sys/fs/cgroup/my-entrypoint
    echo "$" > /sys/fs/cgroup/my-entrypoint/cgroup.procs
fi
If this step is not performed before the buildkitd process starts up, buildkitd will be unable to initialize it's own cgroup (due to the container's root cgroup already having processes directly under it), and will fail with the error: sh: write error: Resource busy.
An important note about running the image
When running the built image in your CI of choice, if you're not using a remote daemon, Earthly will start Buildkit within the same container. In this case, it is important to ensure that the directory used by Buildkit to cache the builds is mounted as a Docker volume. Failing to do so may result in excessive disk usage, slow builds, or Earthly not functioning properly.
Important
We strongly recommend using a Docker volume for mounting /tmp/earthly. If you do not, Buildkit can consume excessive disk space, operate very slowly, or it might not function correctly.
In some environments, not mounting /tmp/earthly as a Docker volume results in the following error:
--> WITH DOCKER RUN --privileged ...
...
rm: can't remove '/var/earthly/dind/...': Resource busy
In EKS, users reported that mounting an EBS volume, instead of a Kubernetes emptyDir worked.
This part of our documentation needs improvement. If you have a Kubernetes-based setup, please let us know how you have mounted /tmp/earthly and whether WITH DOCKER worked well for you.
For more information, see the documentation for earthly/earthly on DockerHub.
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Outline
Introduction
Getting Started
Extending The earthly/earthly Image
Adding Earthly To An Existing Image
cgroups v2 Considerations
An important note about running the image