Treat an uppercase first component as a registry host in split_repo_name

[semx]

split_repo_name decides whether the first component of an image reference is a registry host or a Docker Hub path. It treated the first component as a host only when it contains . or :, or equals localhost.

The canonical reference grammar the daemon uses (distribution/reference splitDockerDomain) also treats the first component as a domain when it contains an uppercase letter, because image path components are always lowercase. docker-py's detection was therefore a strict subset of the daemon's.

As a result, a reference like MyRegistry/image was resolved as a Docker Hub path (docker.io, MyRegistry/image) instead of the registry MyRegistry. Since resolve_repository_name selects which stored credentials get attached (X-Registry-Auth) while the daemon routes on the full name, the auth config chosen did not match the registry the daemon actually contacts.

This aligns split_repo_name with the daemon by also treating a first component containing an uppercase letter as a registry host.

Before:

>>> auth.resolve_repository_name('MyRegistry/image')
('docker.io', 'MyRegistry/image')

After:

>>> auth.resolve_repository_name('MyRegistry/image')
('MyRegistry', 'image')

Added unit tests for MyRegistry/image and MyRegistry/username/image; existing ResolveRepositoryNameTest cases still pass.