The tool does actually check whether you have any uncommitted changes in Git, and fails in that case. So you're protected from that particular mistake. You're not protected from mistakes related to running the tests or checking their results, though, because the tool has nothing to do with that.

The Docker CLI is supposed to support caching on GitHub Actions (https://docs.docker.com/build/cache/backends/gha/) but I suppose I haven't checked how fast it is in practice.