I just ran into this with GitHub Actions and caching.
I believe it’s that _build/test/lib/ranch/ebin/ is a symlink to deps/ranch/ebin/ and during deps.compile it will actually compile the .beam files into the deps/ranch/ebin folder. If there is a discrepancy between architectures, erlang versions, or anything that would cause the .beam file in the deps/ranch/ebin file to be considered invalid for the final runtime container, it will fail to find it. Or I guess if you’re only moving the _build and not the deps too, that’d do it. In that case, if you’re only moving the _build, then you probably want a release. If this isn’t for a release container, then you’ll need to copy deps and _build
Obviously my scenario is very different, but I hope that helps some?