Preserve dependency caches across release versions and preserve build argument comments.
Changing BUILD_VERSION invalidated OS-package and Python-dependency installation layers in both Docker stages, even when the dependencies are unchanged. Docker makes an in-scope build argument available to subsequent RUN instructions, so declaring it early makes a release-version change affect those cache keys.
With the change, the second build reuses the builder OS-package, locked Python-dependency and runtime OS-package layers. Without the change, all three installation steps would run again.
Co-authored-by: David Siegel <dsiegel@MacBook-Air-von-David.local>
