Checksums for Docker Hub actions

[ericcornelissen]

Relates to #49, #215

Summary

Per the GitHub Actions workflow syntax documentation a uses: value may be a "Docker Hub action" (not to be confused with "Docker container action"). As of #214 these are ignored by ghasum. However, it would make sense to include them because they're external artifacts that are pulled in at runtime and my change from one run to the next.