Backup by volumes, not containers

Wouldn't it be nice to allow specifying what to backup on a volume level, not on a container one? Some volumes are shared between containers, and keeping track of where to backup which volume consumes mental capacity :)

I'd like to be able to configure my backups like this:

services:
  postgres:
    image: postgres:11
    labels:
      stack-back.postgres: true
    volumes:
      - pgdata:/var/lib/postgresql/data
...

volumes:
  pgdata:
  shared-state:
    labels:
      stack-back.backup: true
  my-files:
    # instead of bind mount, to specify labels
    labels:
      stack-back.backup: true
    driver_opts:
      type: none
      o: bind
      device: /srv/data

No file or test is named. Start by tracing how stack-back currently discovers container labels and mounted volumes, then determine how Compose volume labels should select backup targets, including shared volumes and the bind-backed volume example. Done means volume-level configuration works without requiring container-level backup labels.