Creating a Docker container action

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

In this article:

Introduction

The action you build will print "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.

Prerequisites

Before you begin, you'll need to create a GitHub repository.

  1. Create a new repository on GitHub. You can choose any repository name or use "hello-world-docker-action" like this example. For more information, see "Create a new repository."

  2. Clone your repository to your computer. For more information, see "Cloning a repository."

  3. From your terminal, change directories into your new repository.

    cd hello-world-docker-action

Create a Dockerfile

In your new hello-world-docker-action directory, create a new Dockerfile file.

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"]

Create 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

# 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 must declare the input using inputs and pass the input in the args keyword.

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

Write 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 use a specific syntax in the form ##[set-output name=<output name>]<value>. For more information, see "Development tools for GitHub Actions."

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

  2. Make sure to make you entrypoint.sh file executable:

    chmod +x entrypoint.sh
  3. Add the following code to your entrypoint.sh file.

    entrypoint.sh

    #!/bin/sh -l
    
    echo "Hello $1"
    time=$(date)
    echo ::set-output name=time::$time

    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 an action's status using exit codes."

Create 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

# 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@v1
with:
  who-to-greet: 'Mona the Octocat'

Commit and push your action to GitHub

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

git add action.yml entrypoint.sh Dockerfile README.md
git commit -m "My first action is ready"
git push

Testing out your action in a workflow

Now you're ready to test your action out in a workflow. When an action is in a private repository, the action can only be used in workflows in the same repository. 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.

.github/workflows/main.yml

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@v1
      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.

.github/workflows/main.yml

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@v1
    - uses: ./ # Uses an action in the root directory
    - name: Hello world action step
      id: hello
      uses: actions/hello-world-docker-action@v1
      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. You should see "Hello Mona the Octocat" or the name you used for the who-to-greet input and the timestamp printed in the log.

A screenshot of using your action in a workflow

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