non-UTF8 filename support

Since its creation ostree (current bootc backend) intentionally didn't support non-UTF8 filenames. In theory, we could lift that restriction as we look towards a direct composefs backend.

I personally think it's almost always going to be a mistake to ship non-UTF8 filenames in containers, but just filing this so we have a tracker.

Also, it'd probably be good to add this to `container lint`.