title: How do I use Docker with GitHub Actions?
body: <p>When I create a GitHub Actions workflow file, the example YAML file contains <code>runs-on: ubuntu-latest</code>. <a href="https://help.github.com/en/articles/virtual-environments-for-github-actions#supported-virtual-environments" rel="noreferrer">According to the docs</a>, I only have the options between a couple versions of Ubuntu, Windows Server and macOS X.</p>
  
  <p>I thought GitHub Actions runs inside Docker. How do I choose my Docker image?</p>