Fixes #9409, Infinite redirect loop for docker.io registry

Specifying a registry name of `docker.io` causes an infinite redirect loop in the docker server -- see #9409. And although the code currently rejects `index.docker.io`, the error message is unclear about what exactly was offensive about the name.

This patch causes docker to reject any name with "docker.io" in the registry portion, not just index.docker.io, and clarifies the error message.