Add automation to update example dockerfiles in the unit docs repo

[avahahn]

See: example dockerfiles
See: related issue in unit-docs

We can automate opening a PR to the unit-docs repository to update all of these dockerfiles. Doing so would reduce the complexity of the release process in that we wouldn't have to worry about doing this ourselves. As it stands it appears that we have forgotten to do this since 1.31.1 and these dockerfiles are now super out of date.

As an example of how we could automate this: we could add a new github actions workflow that triggers on a new release tag, and programmatically opens a PR to the docs repository with the updated dockerfiles. In implementing this we would want to make sure that the source repository is not a fork of Unit and that the dockerfiles as they sit in this repository are ready to merge to the docs repository.

[callahad]

• We should go ahead and manually update the files now.
• We should update RELEASE-PROCESS.md in Add RELEASE-PROCESS.md #1435 to have a manual step to update these with every release.
• We should remove that manual step when this automation issue is addressed.