Github action can be used to run tests on your applications when code has been merged in.

The next step once tests are successful is to build a docker image and push this to the repository.

This will require us to create a new workflow file that will run after the tests are successful to build and push your docker image to a repository.

First create a new file in your workflow folder .github/workflows/docker.yml and paste the following in that file.

name: Docker

on:
  push:
    branches: [ master ]

jobs:
  build:
    runs-on: ubuntu-latest
    steps:
    - name: 'Checkout GitHub Action'
        uses: actions/checkout@v2
    - name: Build and push docker
        uses: docker/build-push-action@v1
        with:
          username: ${{ secrets.REGISTRY_USERNAME }}
          password: ${{ secrets.REGISTRY_PASSWORD }}
          repository: DOCKER_REPOSITORY
          tags: latest

This works by first checking out the current repository on pushed into master, by using the actions/checkout@v2 action. The next step will use the docker build and push action docker/build-push-action@v1. This requires 4 parameters the docker hub username, password, repository and tags.

In the code above you can see we're using the secrets parameter for username and password.

username: ${{ secrets.REGISTRY_USERNAME }}
password: ${{ secrets.REGISTRY_PASSWORD }}

To add secrets to your repository in Github is done by selecting Settings -> Secrets then create a new secret for the username and password.

Ensure your repository has a Dockerfile at the root of your project to build the image.

That's it, now commit this file to your repository and on pushes into master, Github will build a push your docker image to the docker hub.