[sdk-platform-java] Centralize implementation for `library_generation.Dockerfile` and its Airlock variant

> We should probably have a way to share the common code between the two, or maybe only keep the airlock one if it can be used locally as well. Otherwise these two files can easily get diverged from each other as they evolve. This is a separate future enhancement though.

_Originally posted by @blakeli0 in https://github.com/googleapis/sdk-platform-java/pull/3726#discussion_r2023417624_