When building registries, we should use skopeo to check if dependent images exist for the given arch.

If not found:

• a warning should be logged in the build
• a README should be written in the registry plugin/devfile folder as replacement for the yaml
• build should continue

This would allow us to more easily drop out functionality that doesn't (yet, or ever) exist for a given platform without having to fork the registry for each supported arch (eg., dotnet on Z)

For devfile registry, script should run something like ./build/scripts/remove-unavailable-containers.sh devfiles/

Similar script for plugin registry would then do something like ./build/scripts/remove-unavailable-containers.sh v3/

Note also that checking registry.redhat.io might not be a sufficient check since some images may exist in OSBS registry but not be public yet... so like in this script:

https://github.com/redhat-developer/codeready-workspaces/blob/master/dependencies/che-plugin-registry/build/scripts/write_image_digests.sh

You might need an extra processing step to fall back to an internal registry as a second check:

https://github.com/redhat-developer/codeready-workspaces/blob/master/dependencies/che-plugin-registry/build/scripts/write_image_digests_alternate_urls.sh

(why two files? easier to split upstream from downstream code)