> Isn't compatibility issues a major problem for alternative registries?

Er, is it? I've used a handful of different registries and never hit anything that even resembled a compatibility problem. Have I just been lucky?

One example that springs to mind is Homebrew/MacPorts. I think they install things differently and might not be interoperable?

They do things differently, but what does that have to do with docker registries?

It’s an example of incompatibility in registries generally, which I thought was the nature of your surprise at its existence or occurrence. I’m not aware of any such incompatibilities among Docker registries, but other issues may affect usage such as rate limits. I’m not sure what the person you were responding to had in mind.

I had waaay too much problem with pushing to GCR, when pushing to a self-hosted GitLab's registry worked.