Backup by volumes, not containers

[lvu]

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

[lawndoc]

I think some people may prefer to label their backups on the services, but I do like the idea of adding this as an option and allowing people to take whichever approach is more intuitive for them.