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>
