Automate Docker build with GitHub Actions

Overview

Teaching: 1 min
Exercises: 1 min

Questions

• How do I build and publish a new release of my application?

Objectives

• Build a Docker image and push it to DockerHub if the tests pass.

• Only build and push when a new release is published,.

You can add a job to your GitHub Action workflow which will automatically build the Docker image and push it to DockerHub if the tests pass.

Build and push Docker image

• Find the Build and push Docker images action on the GitHub Marketplace: https://github.com/marketplace
• Use the action to automatically build and push a Docker image with tag latest if the tests pass. Do the build in a new job named build. Use the Example usage section in the action readme.
• You can safely store your DockerHub username and password using secrets.
  • Go to the ⚙️ Settings tab of your GitHub repository
  • Go to Secrets in the left navbar.
  • Create 2 secrets that will be used by the GitHub Actions workflow: DOCKER_USERNAME and DOCKER_PASSWORD 🔒

Once a secret has been defined no one (not even you) can read it 🙈, you can still override it though.

Solution

name: Test and publish to DockerHub
on:
  push:
    branches: [ master ]
jobs:
  test:
    [...]
  
  build:
    needs: test
    runs-on: ubuntu-latest
   
 steps:
       - uses: docker/build-push-action@v1
      with:
        username: ${{ secrets.DOCKER_USERNAME }}
        password: ${{ secrets.DOCKER_PASSWORD }}
        repository: myorg/my-repository
        tags: latest

It is recommended to avoid rebuilding and publishing a new image at every new push to master. We will now set the build and push job to only be triggered if a new release is created on GitHub (a.k.a tag 🏷️)

Build and push image only when release

• Add a condition to only push to DockerHub when a release is pushed
• Use the GitHub tag to tag the DockerHub image
• 2 solutions are available for this case:
  • using a if condition in the GitHub workflow (recommended)
  • use a parameter provided by the “Build and push Docker images” action.

Solution

name: Test and publish to DockerHub
on:
push:
 branches: [ master ]
jobs:
test:
 [...]

build:
 if: startsWith(github.event.ref, 'refs/tags')
 needs: test
 runs-on: ubuntu-latest

 steps:
    - uses: docker/build-push-action@v1
      with:
        username: ${{ secrets.DOCKER_USERNAME }}
        password: ${{ secrets.DOCKER_PASSWORD }}
        repository: myorg/my-repository
        tag_with_ref: true

Key Points

• Use the official GitHub Action from Docker.

• Set sensible informations, such as passwords, in GitHub Secrets.