The stock image has no libguestfs, so UrBackup disables image mounting and
single files can't be restored from image backups. Add libguestfs-tools +
kernel/supermin/qemu on top of uroni/urbackup-server:2.5.x, built by Gitea
Actions into registry.ginnoir.com/ginnoir/urbackup-server (Portainer only
pulls). Verified on valhalla: KVM-accelerated appliance, MOUNT TEST OK.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
