GitHub integration

With the public release of GitHub Actions, we thought that there might be value in adding a container structure test action for consumption in workflows. We've personally found value in wiring it up in our build container repositories to check our build tools actually run before pushing to the container registry.

Would there be interest in putting a link to the marketplace (https://github.com/marketplace/actions/container-structure-test-action) in the container-structure-test repository?