With new changes in clean up alpine cruft from downstream [1], we should ensure that we're actually using upstream assets in the registry build process from mid/downstream.

Thus where che-devfile-registry and che-plugin-registry contain rhel.* files, we should sync & transform them into what they need to be in downstream.

[1] https://github.com/redhat-developer/codeready-workspaces/pull/432/

Should also look at keep_only_latest.sh and decide if it's still useful – does it do anything of value now that we only include a single version of each plugin in the registry?