I think it would be valuable if Google were to provide volumes in Cloud Build of package registries (e.g. Go Modules; PyPi; Maven; NPM etc.).
Google provides a mirror of a subset of Docker Hub. This confers several benefits: Google’s imprimatur; speed (latency); bandwidth; and convenience.
The same benefits would apply to package registries.
In the meantime, there’s a hacky way to gain some of the benefits of these when using Cloud Build.