(Cron) Preview Generator support

[joshtrichards]

Allow the user to enable and configure a cron entry for the NC Preview Generator.

To do so, add CRON_PREVIEW={hour} (allowed values for hour are 0-23 since crond uses 24H system) in the startup environment parameters for the cron service container.

For example, if your Docker environment uses UTC time the following will run the Preview Generator daily at 2:00 AM Eastern Time (also known as 6:00 AM UTC):

  cron:
    depends_on:
      - db
      - redis
      - app
    image: nextcloud:24-apache
    restart: unless-stopped
    environment:
      - CRON_PREVIEW=6
    volumes:
      - app-var-www-html:/var/www/html
    entrypoint: /cron.sh
    networks:
      - app-network

User must install the Preview Generator app ahead of time from the app store.

[J0WI]

I'd prefer #1964 for a generic way to extend the image for 3rd party apps.

[J0WI]

Closing in favour of #1964.