[sdk-platform-java] showcase upgrade golden files does not run out of box for local dev

When attempting to update golden files according to readme for https://github.com/googleapis/sdk-platform-java/pull/3810:
https://github.com/googleapis/sdk-platform-java/blob/49a7ae50071e75fe0d161a4eb9360a4fe4e6147b/java-showcase/README.md?plain=1#L94

Does not work for my local development out of box:
1. docker build and run. Run docker command without "sudo" is discouraged (go/installdocker#sudoless-docker )
https://github.com/googleapis/sdk-platform-java/blob/49a7ae50071e75fe0d161a4eb9360a4fe4e6147b/java-showcase/scripts/generate_showcase.sh#L56
https://github.com/googleapis/sdk-platform-java/blob/49a7ae50071e75fe0d161a4eb9360a4fe4e6147b/java-showcase/scripts/generate_showcase.sh#L76
3. COPY step in Dockerfile. These two lines gives me "COPY failed: no source files were specified". I was able to get pass errors and successfully update files by removing these two lines. Though I am not familiar with it to determine if they are truely redundant?
https://github.com/googleapis/sdk-platform-java/blob/49a7ae50071e75fe0d161a4eb9360a4fe4e6147b/.cloudbuild/library_generation/library_generation.Dockerfile#L74
https://github.com/googleapis/sdk-platform-java/blob/49a7ae50071e75fe0d161a4eb9360a4fe4e6147b/.cloudbuild/library_generation/library_generation.Dockerfile#L79

my local docker version:
Docker version 20.10.21

Need more context to determine if need fix to these, or updates in readme are sufficient.