feat(backup): exclude a volume by name, not only by image

volume_is_fully_ignored can only skip a volume when every container using it is ignored, so a container holding a derived tree next to state that must be kept cannot express the exclusion at all. The matrix docker-in-docker runner is exactly that: matrix_mdad_docker, matrix_mdad_matrix and matrix_mdad_state all hang off one container, and the derived one is an inner overlay2 store that no rsync in the chain can restore faithfully (none carries -X, so trusted.overlay.* is stripped in both directions).

--volumes-no-backup-required names volumes directly. The check runs before containers_using_volume, so an excluded volume costs no docker call and the decision no longer depends on which containers happen to exist at backup time.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/baudolo/backup/app.py

@@ -54,6 +54,14 @@ def main() -> int:
 
         for volume_name in docker_volume_names():
             print(f"Start backup routine for volume: {volume_name}", flush=True)
+
+            if volume_name in args.volumes_no_backup_required:
+                print(
+                    f"Skipping volume '{volume_name}' entirely (declared no-backup).",
+                    flush=True,
+                )
+                continue
+
             containers = containers_using_volume(volume_name)
 
             if volume_is_fully_ignored(containers, args.images_no_backup_required):

src/baudolo/backup/cli.py

@@ -68,6 +68,13 @@ def parse_args() -> argparse.Namespace:
         help="Exact image references (repo:tag, incl. any registry prefix) for which no backup should be performed",
     )
 
+    p.add_argument(
+        "--volumes-no-backup-required",
+        nargs="+",
+        default=[],
+        help="Exact volume names that are never backed up, whatever containers use them. For derived trees a restore cannot reproduce, above all a nested docker data root",
+    )
+
     p.add_argument(
         "--everything",
         action="store_true",