Creating a Docker container action

This guide shows you the minimal steps required to build a Docker container action.

In this article

Introduction

In this guide, you'll learn about the basic components needed to create and use a packaged Docker container action. To focus this guide on the components needed to package the action, the functionality of the action's code is minimal. The action prints "Hello World" in the logs or "Hello [who-to-greet]" if you provide a custom name.

Once you complete this project, you should understand how to build your own Docker container action and test it in a workflow.

Self-hosted runners must use a Linux operating system and have Docker installed to run Docker container actions. For more information about the requirements of self-hosted runners, see About self-hosted runners.

Warning

When creating workflows and actions, you should always consider whether your code might execute untrusted input from possible attackers. Certain contexts should be treated as untrusted input, as an attacker could insert their own malicious content. For more information, see Security hardening for GitHub Actions.

Prerequisites

Creating a Dockerfile

In your new hello-world-docker-action directory, create a new Dockerfile file. Make sure that your filename is capitalized correctly (use a capital D but not a capital f) if you're having issues. For more information, see Dockerfile support for GitHub Actions.

Dockerfile

Dockerfile
# Container image that runs your code
FROM alpine:3.10

# Copies your code file from your action repository to the filesystem path `/` of the container
COPY entrypoint.sh /entrypoint.sh

# Code file to execute when the docker container starts up (`entrypoint.sh`)
ENTRYPOINT ["/entrypoint.sh"]

Creating an action metadata file

Create a new action.yml file in the hello-world-docker-action directory you created above. For more information, see Metadata syntax for GitHub Actions.

action.yml

YAML
# action.yml
name: 'Hello World'
description: 'Greet someone and record the time'
inputs:
  who-to-greet:  # id of input
    description: 'Who to greet'
    required: true
    default: 'World'
outputs:
  time: # id of output
    description: 'The time we greeted you'
runs:
  using: 'docker'
  image: 'Dockerfile'
  args:
    - ${{ inputs.who-to-greet }}

This metadata defines one who-to-greet input and one time output parameter. To pass inputs to the Docker container, you should declare the input using inputs and pass the input in the args keyword. Everything you include in args is passed to the container, but for better discoverability for users of your action, we recommended using inputs.

GitHub will build an image from your Dockerfile, and run commands in a new container using this image.

Writing the action code

You can choose any base Docker image and, therefore, any language for your action. The following shell script example uses the who-to-greet input variable to print "Hello [who-to-greet]" in the log file.

Next, the script gets the current time and sets it as an output variable that actions running later in a job can use. In order for GitHub to recognize output variables, you must write them to the $GITHUB_OUTPUT environment file: echo "<output name>=<value>" >> $GITHUB_OUTPUT. For more information, see Workflow commands for GitHub Actions.

  1. Create a new entrypoint.sh file in the hello-world-docker-action directory.

  2. Add the following code to your entrypoint.sh file.

    entrypoint.sh

    Shell
    #!/bin/sh -l

echo "Hello $1"
time=$(date)
echo "time=$time" >> $GITHUB_OUTPUT

    If entrypoint.sh executes without any errors, the action's status is set to success. You can also explicitly set exit codes in your action's code to provide an action's status. For more information, see Setting exit codes for actions.

  3. Make your entrypoint.sh file executable. Git provides a way to explicitly change the permission mode of a file so that it doesn’t get reset every time there is a clone/fork.

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

  4. Optionally, to check the permission mode of the file in the git index, run the following command.

    Shell
    git ls-files --stage entrypoint.sh

    An output like 100755 e69de29bb2d1d6434b8b29ae775ad8c2e48c5391 0 entrypoint.sh means the file has the executable permission. In this example, 755 denotes the executable permission.

Creating a README

To let people know how to use your action, you can create a README file. A README is most helpful when you plan to share your action publicly, but is also a great way to remind you or your team how to use the action.

In your hello-world-docker-action directory, create a README.md file that specifies the following information:

  • A detailed description of what the action does.
  • Required input and output arguments.
  • Optional input and output arguments.
  • Secrets the action uses.
  • Environment variables the action uses.
  • An example of how to use your action in a workflow.

README.md

Markdown
# Hello world docker action

This action prints "Hello World" or "Hello" + the name of a person to greet to the log.

## Inputs

## `who-to-greet`

**Required** The name of the person to greet. Default `"World"`.

## Outputs

## `time`

The time we greeted you.

## Example usage

uses: actions/hello-world-docker-action@v2
with:
  who-to-greet: 'Mona the Octocat'

Commit, tag, and push your action to GitHub

From your terminal, commit your action.yml, entrypoint.sh, Dockerfile, and README.md files.

It's best practice to also add a version tag for releases of your action. For more information on versioning your action, see About custom actions.

Shell
git add action.yml entrypoint.sh Dockerfile README.md
git commit -m "My first action is ready"
git tag -a -m "My first action release" v1
git push --follow-tags

Testing out your action in a workflow

Now you're ready to test your action out in a workflow.

  • Public actions can be used by workflows in any repository.

Example using a public action

The following workflow code uses the completed hello world action in the public actions/hello-world-docker-action repository. Copy the following workflow example code into a .github/workflows/main.yml file, but replace the actions/hello-world-docker-action with your repository and action name. You can also replace the who-to-greet input with your name. Public actions can be used even if they're not published to GitHub Marketplace. For more information, see Publishing actions in GitHub Marketplace.

.github/workflows/main.yml

YAML
on: [push]

jobs:
  hello_world_job:
    runs-on: ubuntu-latest
    name: A job to say hello
    steps:
      - name: Hello world action step
        id: hello
        uses: actions/hello-world-docker-action@v2
        with:
          who-to-greet: 'Mona the Octocat'
      # Use the output from the `hello` step
      - name: Get the output time
        run: echo "The time was ${{ steps.hello.outputs.time }}"

Example using a private action

Copy the following example workflow code into a .github/workflows/main.yml file in your action's repository. You can also replace the who-to-greet input with your name. This private action can't be published to GitHub Marketplace, and can only be used in this repository.

.github/workflows/main.yml

YAML
on: [push]

jobs:
  hello_world_job:
    runs-on: ubuntu-latest
    name: A job to say hello
    steps:
      # To use this repository's private action,
      # you must check out the repository
      - name: Checkout
        uses: actions/checkout@v4
      - name: Hello world action step
        uses: ./ # Uses an action in the root directory
        id: hello
        with:
          who-to-greet: 'Mona the Octocat'
      # Use the output from the `hello` step
      - name: Get the output time
        run: echo "The time was ${{ steps.hello.outputs.time }}"

From your repository, click the Actions tab, and select the latest workflow run. Under Jobs or in the visualization graph, click A job to say hello.

Click Hello world action step, and you should see "Hello Mona the Octocat" or the name you used for the who-to-greet input printed in the log. To see the timestamp, click Get the output time.

Accessing files created by a container action

When a container action runs, it will automatically map the default working directory (GITHUB_WORKSPACE) on the runner with the /github/workspace directory on the container. Any files added to this directory on the container will be available to any subsequent steps in the same job. For example, if you have a container action that builds your project, and you would like to upload the build output as an artifact, you can use the following steps.

workflow.yml

YAML
jobs:
  build:
    runs-on: ubuntu-latest
    steps:
      - name: Checkout
        uses: actions/checkout@v4

      # Output build artifacts to /github/workspace on the container.
      - name: Containerized Build
        uses: ./.github/actions/my-container-action

      - name: Upload Build Artifacts
        uses: actions/upload-artifact@v4
        with:
          name: workspace_artifacts
          path: ${{ github.workspace }}

For more information about uploading build output as an artifact, see Storing and sharing data from a workflow.

Example Docker container actions on GitHub.com

You can find many examples of Docker container actions on GitHub.com.