Add possibility to handle multiple network tags

[Murazaki]

Very concise update, but basically this gives the possibility of handling different network access depending on the docker service.

The user would need to expose the corresponding files for each group :

volumes:
      - ./network_internal.conf:/etc/nginx/network_internal.conf
      - ./network_group0.conf:/etc/nginx/network_group0.conf
      - ./network_group1.conf:/etc/nginx/network_group1.conf
      - ./network_group2.conf:/etc/nginx/network_group2.conf
      - ./network_group3.conf:/etc/nginx/network_group3.conf

Just sharing that as I've been using it. Let me know if that helps anybody.