`docker.io` should be referenced to `registry-1.docker.io`

[jonesbusy]

Description of the bug

https://github.com/oras-project/oras-go/blob/d92df9df65805259b6f3b8114d9dc939d71f5264/registry/reference_test.go#L235-L249

https://github.com/oras-project/oras-py/blob/1790ad7df3c83884f24aabd2ab4b2d97e4c2f3b0/oras/defaults.py#L10-L15