crane auth login: fail to login to ECR with repository name attached

### Describe the bug

When I try to login into ECR using crane with the command `crane auth login --username AWS --password-stdin 123456789.ecr.us-east-1.amazonaws.com/my_repository` I get an error message `Error: registries must be valid RFC 3986 URI authorities: 123456789.ecr.us-east-1.amazonaws.com/my_repository`.

This might be more of a QoL feature than a bug fix :)

### Expected behavior

I expect `crane auth login` to ignore the repository name, similarly to what `docker login` does.

### Additional context

`crane version` -> `0.20.2`
Registry used -> ECR

Start at the crane auth login command and trace how its registry argument is validated when a repository name is attached to an ECR host. Reproduce the command from the issue, then verify that login accepts the repository-qualified input while preserving validation for invalid registry authorities.

Written by the indexing model from the issue text.