Make backup dir configurable

[claudiu-cristea]

Problem

I have this case: I need to make container's /data dir a persistent/named volume in order to persist Virtuoso storage. For performance reasons I don't want to mount this dir in the host machine. I'm using this setup in docker-compose.yml:

services:
  virtuoso:
    ...
    volumes:
      - source: virtuoso
        target: /data
        type: volume
        volume:
          nocopy: true
...
volumes:
  # Persistent Docker volume for Virtuoso data.
  virtuoso:
    name: virtuoso

But the container's /data/backups directory I want to mount on the host machine. Unfortunately, I this doesn't work:

    volumes:
      - source: virtuoso
        target: /data
        type: volume
        volume:
          nocopy: true
      - ./.backups:/data/backups

See https://stackoverflow.com/questions/52137210/how-to-share-sub-folders-from-a-docker-named-volume why...

Proposal

Allow to configure the backups directory via an environment variable.