feat(devicebackup): build urbackup-server with guestmount for image file restores

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>

stacks/devicebackup/urbackup/Dockerfile

@@ -0,0 +1,17 @@
+# UrBackup server + libguestfs, so the web UI can mount image backups and restore
+# single files out of them. The stock image ships without guestmount, and the server
+# logs "Image mounting disabled: TEST FAILED: guestmount is missing".
+#
+# libguestfs boots a tiny helper VM (supermin appliance) to read the VHDZ, which needs
+# a kernel in /boot plus /dev/kvm and /dev/fuse at runtime (see docker-compose.yml).
+# Built by .gitea/workflows/build-urbackup-server.yml → registry.ginnoir.com.
+ARG URBACKUP_TAG=2.5.x
+FROM uroni/urbackup-server:${URBACKUP_TAG}
+
+RUN apt-get update \
+ && DEBIAN_FRONTEND=noninteractive apt-get install -y --no-install-recommends \
+      libguestfs-tools linux-image-amd64 supermin qemu-system-x86 \
+ && rm -rf /var/lib/apt/lists/*
+
+# Use the direct (qemu) backend; there is no libvirt in the container.
+ENV LIBGUESTFS_BACKEND=direct