Improve heuristic for opening docker-library/official-images PRs

This pull request is a good example of why our current heuristic for whether or not we should automatically open a PR in docker-library/official-images is overly simplistic.

Currently we only open the official-images PR if the generated manifest file changes somewhere other than the git SHA.

However, PR #232 fixes a bug in the lein images but only bumps the git SHA in the manifest. It probably still makes sense to not always open an official-images PR on every change we merge here, though. But maybe just doing that is the simplest and most reliable option?

One option would be to build (some of*) the images in our CI pipeline here and detect when they change and base the decision on that. That would be tricky and slow, but is probably the most correct heuristic.

Another option would be to detect a string in the git commit message that forces an official-images PR (e.g. [force-official-images-pr]). We would just have to remember to add it when we needed it.

* It would likely be slow and unreliable to build every architecture in GitHub Actions' free tier infrastructure.

Start by tracing the current generated-manifest check that decides whether to open an official-images PR, then compare its behavior with docker-clojure PR #232. Review the proposed CI image builds and commit-message override approaches; done means agreeing on a reliable heuristic and defining how its decision is verified.