How do I use Docker with GitHub Actions?

GitHub actions provision a virtual machine – as you noted, either Ubuntu, Windows or macOS – and run your workflow inside of that. You can then use that virtual machine to run a workflow inside a container.

Use the container specifier to run a step inside a container. Be sure to specify runs-on as the appropriate host environment for your container (ubuntu-latest for Linux containers, windows-latest for Windows containers). For example:

jobs:
  vm:
    runs-on: ubuntu-latest
    steps:
      - run: |
          echo This job does not specify a container.
          echo It runs directly on the virtual machine.
        name: Run on VM
  container:
    runs-on: ubuntu-latest
    container: node:10.16-jessie
    steps:
      - run: |
          echo This job does specify a container.
          echo It runs in the container instead of the VM.
        name: Run in container