install: Add `--copy-container-credentials`

We document that registry credentials are honored from /etc/ostree/auth.json, but it's easy to miss (and needs to highlighted much better) (there's also the general issue with embedding the pull secret in the image itself, cc https://github.com/containers/bootc/issues/22 )

Now when using bootc install to-filesystem with a private registry, we could add bootc install --copy-container-credentials where we go and slurp out ~/.config/containers/auth.json and inject it into the final system as /etc/ostree/auth.json.

This way we get a flow where we

• podman login on the original host
• bootc install to-filesystem --copy-container-credentials

And the original podman credentials (injected into ~/.config/containers/auth.json) could have come from e.g. cloud-init (which is arguably more secure than embedding them into the image itself).