[FEATURE] Automatically lowercase `registry` input

[strugee]

Is your feature request related to a problem? Please describe.

If your GitHub organization name includes uppercase characters and you try to log into GitHub Container Registry, podman login errors out:

Error: parse reference from "ghcr.io/<org name>": invalid reference format: repository name must be lowercase

This is annoying to fix without introducing a new action literally just to lowercase this string.

Describe the solution you'd like

The action should automatically lowercase the registry input, because it's basically never going to be valid for this to include capital letters due to Docker image naming restrictions - I think. Someone should confirm for sure.

Describe alternatives you've considered

A boolean lowercase_registry parameter that does exactly what it sounds like.

Additional context