Add support for reading credential from podman's auth.json

[grimmy]

Podman stores registry credentials in $XDG_RUNTIME_DIR/containers/auth.json which is in the exact same format as Docker's config.json. I currently just symlinked $XDG_RUNTIME_DIR/containers/auth.json to ~/.docker/config.json and everything is working fine. But it would be cool if reg could just check this file itself.