Move matrix-nginx-proxy to its own container network

This service will be removed soon, but for now we need to get rid of
`matrix_docker_network` usage everywhere.

roles/custom/matrix-nginx-proxy/tasks/ssl/setup_ssl_lets_encrypt_obtain_for_domain.yml

@@ -66,7 +66,7 @@
     --user={{ matrix_user_uid }}:{{ matrix_user_gid }}
     --cap-drop=ALL
     -p 127.0.0.1:{{ matrix_ssl_lets_encrypt_certbot_standalone_http_port }}:8080
-    --network={{ matrix_docker_network }}
+    --network={{ matrix_nginx_proxy_container_network }}
     --mount type=bind,src={{ matrix_ssl_config_dir_path }},dst=/etc/letsencrypt
     --mount type=bind,src={{ matrix_ssl_log_dir_path }},dst=/var/log/letsencrypt
     {{ matrix_ssl_lets_encrypt_certbot_docker_image }}