execute commands before and after backup

[wehrstedt]

Add a possibility to run a command before and after the backup process. As a concrete example, let's assume that you wan't to backup a nextcloud instance:

version: '2'

volumes:
  nextcloud:
  nextcloud_data:
  db:

services:
  db:
    image: mariadb
    restart: always
    command: --transaction-isolation=READ-COMMITTED --binlog-format=ROW
    volumes:
      - db:/var/lib/mysql
    environment:
      - MYSQL_ROOT_PASSWORD=root
      - MYSQL_PASSWORD=root
      - MYSQL_DATABASE=nextcloud
      - MYSQL_USER=nextcloud
    labels:
      restic-compose-backup.mariadb: true
      # run occ maintenance:mode --on as user www-data in container nextcloud before backup
      restic-compose-backup.before-backup.priority: 1
      restic-compose-backup.before-backup.command: ./occ maintenance:mode --on
      restic-compose-backup.before-backup.user: www-data
      restic-compose-backup.before-backup.service-name: nextcloud

  nextcloud:
    image: nextcloud
    restart: always
    links:
      - db
    volumes:
      - nextcloud:/var/www/html
      - nextcloud_data:/var/lib/nextcloud
    environment:
      - MYSQL_PASSWORD=root
      - MYSQL_DATABASE=nextcloud
      - MYSQL_USER=nextcloud
      - MYSQL_HOST=db
      - NEXTCLOUD_DATA_DIR=/var/lib/nextcloud
      - NEXTCLOUD_TRUSTED_DOMAINS="www.fuubar.com"
    labels:
      restic-compose-backup.volumes: true
      # run occ maintenance:mode --on as user www-data in container nextcloud after backup
      restic-compose-backup.before-backup.priority: 2
      restic-compose-backup.after-backup.command: ./occ maintenance:mode --off
      restic-compose-backup.after-backup.user: www-data
      restic-compose-backup.after-backup.service-name: nextcloud

This example will first start backup the database (priority=1). Before the backup starts, it will execute occ maintenance:mode --on as user www-data in container nextcloud. After that, the volumes of the nextcloud-container will be processed (priority=2) and after that disables the maintenance mode.