Add squid as a forwarding proxy so containers are exposed through a VPN

Implementation isn't perfect, but as `qbittorrent` already had an outbound VPN connection, it makes sense to just reuse it.

ansible/roles/pve_docker/files/synapse/docker-compose.yml

@@ -10,6 +10,8 @@ services:
       - SYNAPSE_REPORT_STATS=yes
       - UID={{ docker_user.id }}
       - GID={{ docker_user.id }}
+      - HTTP_PROXY={{ pve_hosts.qbittorrent.ip }}
+      - HTTPS_PROXY={{ pve_hosts.qbittorrent.ip }}
     volumes:
       - "{{ app_data_dir }}/synapse/homeserver.yaml:/etc/homeserver.yaml"
       - "{{ app_data_dir }}/synapse:/data"