GitHub Docker Container Action

Build, package, and test custom GitHub Actions using Docker containers.

Not what you need? For running sidecar services (Redis, PostgreSQL, etc.)
in CI workflows, see the github-service-containers skill.

Prerequisites

• Repository on GitHub (public, internal, or private)
• Basic understanding of GitHub Actions and Docker
• Self-hosted runners must run Linux with Docker installed

Security: Always treat workflow inputs as untrusted. Avoid script injection
via ${{ }} in run: blocks.

Workflow: Creating a Docker Action

Step 1: Create project structure

my-action/
├── Dockerfile
├── action.yml
├── entrypoint.sh
└── README.md

Step 2: Write Dockerfile

See [dockerfile-patterns.md](references/dockerfile-patterns.md)

Minimal:

FROM alpine:3.21
COPY entrypoint.sh /entrypoint.sh
ENTRYPOINT ["/entrypoint.sh"]

Step 3: Define action metadata

See [action-metadata.md](references/action-metadata.md)

Declare inputs, outputs, and Docker configuration in action.yml.

Step 4: Write entrypoint script

See [entrypoint-scripts.md](references/entrypoint-scripts.md)

Script receives inputs as positional args. Write outputs to $GITHUB_OUTPUT.

Step 5: Make entrypoint executable

git add entrypoint.sh
git update-index --chmod=+x entrypoint.sh

Verify: git ls-files --stage entrypoint.sh should show 100755.

Step 6: Tag and push

git add action.yml entrypoint.sh Dockerfile README.md
git commit -m "Initial action release"
git tag -a -m "v1 release" v1
git push --follow-tags

Step 7: Test in a workflow

See [workflow-testing.md](references/workflow-testing.md)

Quick Reference

Key environment variables

Container filesystem mapping

The runner maps GITHUB_WORKSPACE to /github/workspace in the container. Files written there are available to subsequent workflow steps.

Reading Order

In This Reference