Running Buildkite Agent with Docker

You can run the Buildkite Agent inside a Docker container using the official image on Docker Hub.

Running each build in it’s own container

These instructions cover how to run the agent using Docker. If you want to learn how to isolate each build using Docker and any of our standard Linux-based installers read the Containerized Builds with Docker guide.

Running via Docker

Start an agent with the official image based on Alpine Linux:

docker run -d -t --name buildkite-agent buildkite/agent:3 start --token "<your-agent-token>"

A much larger Ubuntu-based image is also available:

docker run -d -t --name buildkite-agent buildkite/agent:3-ubuntu start --token "<your-agent-token>"

Caveats for builds that need Docker access.

If your build jobs require Docker access, and you’re passing through the Docker socket, you must ensure the build path is consistent between the Docker host and the agent container. See Allowing builds to use Docker for more details..

Version tagging

The default tag (i.e. buildkite/agent:latest) will always point to the latest stable release, but we recommend you use buildkite/agent:3 to prevent breaking changes. If you want to use an exact version, you can use the corresponding tag, such as buildkite/agent:3.0.1. See Docker Hub for a list of all the available versions.

Default file locations

  • Configuration: /buildkite/buildkite-agent.cfg
  • Hooks: /buildkite/hooks
  • Builds: /buildkite/builds
  • Agent user home: /root

Configuration

Most agent configuration settings can be set with environment variables. You can also mount a configuration file in, for example:

docker run \
  -v "/path/to/buildkite-agent.cfg:/buildkite/buildkite-agent.cfg:ro" \
  -d \
  -t \
  --name buildkite-agent \
  buildkite/agent:3 start --token "<your-agent-token>"

Adding hooks

You can add custom agent hooks by mounting or copying them into the /buildkite/hooks directory, and ensuring they are executable.

For example, this is how you'd mount the hooks directory using a read-only host volume:

docker run \
  -v "/path/to/buildkite-hooks:/buildkite/hooks:ro" \
  -d \
  -t \
  --name buildkite-agent \
  buildkite/agent:3 start --token "<your-agent-token>"

Alternatively, if you create your own image based off buildkite/agent, you can copy your hooks into the correct location:

FROM buildkite/agent:3

COPY hooks /buildkite/hooks/

Permissions errors when using Docker

A problem you may encounter when using Docker volume mounts (-v) in Linux or Windows is that the container may create files on the host system with root user permissions. This can result in errors like the following:

$ git clean -fxdq
warning: failed to remove dist/

Permissions on the host are set based on the user running the Docker daemon, which under Linux is generally root. When the Agent (running as buildkite-agent) tries to subsequently remove or modify those files, permissions errors occur.

To ensure correct file permissions, you can:

  • Change the way permissions are set on the files created by your Docker container: modify your container’s USER or modify your build commands.

  • Configure user namespace remapping on your Docker host to ensure that container users are remapped to the same user running your buildkite-agent.

  • Run a script before or after builds that resets permissions. You can do this either via Docker (because it runs as root) or via sudo. See the Buildkite Elastic Stack for AWS’s fix-buildkite-agent-builds-permissions script or the sudoers.conf script for examples of using an agent hook and sudo command to reset permissions.

Allowing builds to use Docker

To use Docker and volume mounting from build scripts, you need to ensure that the builds directory, and the buildkite-agent binary path, are mounted in from the host machine, and that their paths on the host and in the agent container are the same.

For example, when a script runs the command docker run --volume "$PWD:/code" ... the $PWD environment variable will resolve to the path in your agent’s container (e.g. /var/lib/buildkite/builds/my-org/my-pipeline). The Docker daemon, which exists on the host machine, will attempt to mount /var/lib/buildkite/builds/my-org/my-pipeline from the host filesystem, not the agent container filesystem. If that directory does not exist on the host, Docker will mount an empty directory to /code without showing any error.

The following example shows how to configure the agent container with the correct host volumes mounts, and BUILDKITE_BUILD_PATH configuration:

docker run \
  -v "/var/lib/buildkite/builds:/var/lib/buildkite/builds" \
  -v "/usr/local/bin/buildkite-agent:/usr/local/bin/buildkite-agent" \
  -v "/var/run/docker.sock:/var/run/docker.sock" \
  -e "BUILDKITE_BUILD_PATH=/var/lib/buildkite/builds" \
  -d \
  -t \
  --name buildkite-agent \
  buildkite/agent:3 start --token "<your-agent-token>"

Security considerations

Providing builds with a Docker socket gives them access to whatever the docker daemon has access to on the host system. Typically this is root, which means builds have full root system access. This can be mitigated somewhat with user namespace remapping, but caution should still be excercised.

Exposing build secrets into the container

There are many approaches to exposing secrets to Docker containers. In addition, many Docker platforms have their own methods for exposing secrets. If you’re running your own Docker containers, we recommend using a read-only host volume.

The following example mounts a directory containing secrets on the host machine ($HOME/buildkite-secrets) into the container as a read-only data volume at /buildkite-secrets:

docker run \
  -v "/path/to/buildkite-secrets:/buildkite-secrets:ro" \
  -d \
  -t \
  --name buildkite-agent \
  buildkite/agent:3 start --token "<your-agent-token>"

If you've exposed pipeline secrets as environment variables, you can pass them through to the container using the -e option:

docker run \
  -e MY_SECRET_ENV \
  -d \
  -t \
  --name buildkite-agent \
  buildkite/agent:3 start --token "<your-agent-token>"

Authenticating private git repositories

To configure a git-credentials file located at /buildkite-secrets/git-credentials, you could use the following environment agent hook mounted to /buildkite/hooks/environment:

#!/bin/bash

set -euo pipefail

git config --global credential.helper "store --file=/buildkite-secrets/git-credentials"

# You can export other secrets here too
# export FOO=bar

To configure a private SSH key located at /buildkite-secrets/id_rsa_buildkite_git you could use the following environment agent hook mounted to /buildkite/hooks/environment:

#!/bin/bash

set -euo pipefail

eval "$(ssh-agent -s)"
ssh-add -k /buildkite-secrets/id_rsa_buildkite_git

# You can export other secrets here too
# export FOO=bar

Other options for configuring Git and SSH include:

  • Running ssh-agent on the host machine and mounting the ssh-agent socket into the containers. See the Buildkite Agent SSH keys documentation for examples on using ssh-agent.
  • The least-secure approach: the built-in docker-ssh-env-config support allows you to pass in keys via environment variables.

Entrypoint customizations

The entrypoint uses tini to correctly pass signals to, and kill, sub-processes. Instead of redefining ENTRYPOINT we recommend you copy executable scripts into /docker-entrypoint.d/. All executable scripts should not contain any file extension, and will be executed in alphanumeric order.