Add docker compose advanced example

[Marsu31]

URL to the Wiki page

https://github.com/qdm12/gluetun-wiki/blob/main/setup/advanced/vpn-port-forwarding.md#qbittorrent-example

What's incorrect?

Hi !
We could add a more advanced example which uses the configs section in docker compose (version > v2.23.1) :

services:
  gluetun:
    image: qmcgaw/gluetun:latest
    [...]
    environment:
      [...]
      - VPN_PORT_FORWARDING_UP_COMMAND=/usr/local/sbin/up_command.sh {{PORTS}}
    configs:
      - source: up_command
        target: /usr/local/sbin/up_command.sh
        mode: 0500
    [...]

  qbittorrent:
    image: lscr.io/linuxserver/qbittorrent:latest
    [...]

configs:
  up_command:
    content: |
      #!/bin/sh
      # uncomment to debug commands
      #set -x
      echo === up command - begin =========================================================
      # redirect stderr to stdout
      exec 2>&1
      # give a name to arguments
      port=$$1
      # call qbittorrent api
      echo call qbittorrent api to set listening port to $${port}
      /usr/bin/wget -O - --retry-connrefused --post-data "json={\"listen_port\":$${port}}" http://127.0.0.1:${QBITTORRENT_UI_PORT}/api/v2/app/setPreferences
      echo === up command - end ===========================================================
      exit 0

It does the same as the actual short command but some would prefer this clearer syntax.

Regards.

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