This is done for a few reasons:
- less globals and more indepdendence for each role is better. We rely
on various externally-hosted roles and they don't rely on this global
either.
- `matrix_container_global_registry_prefix` could make people think they
could just override this variable and have all their images pull from
elsewhere. This is rarely the case, unless you've taken special care
to mirror all the various components (from their respective
registries) to your own. In such a case, you probably know what you're
mirroring and can adjust individual variables.
- nowadays, various components live on different registries.
With Docker Inc tightening rate limits for Docker Hub, it's even more
likely that we'll see increased diversity in where images are hosted
