Support passing SSH keys as fd / home dir

[omertuc]

I think we could support --root-ssh-authorized-keys-fd or so, and pass it via file descriptor. That'd avoid the bind dance here.

Or maybe we should learn --ssh-home-dir, and directly scrape all keys from there?

Originally posted by @cgwalters in https://github.com/containers/bootc/pull/1063#discussion_r1935611609