safer port binding of etherpad docker container

don't bind to any host port if nginx_proxy is used
only bind to localhost if it's not used

group_vars/matrix_servers

@@ -765,6 +765,8 @@ matrix_dimension_database_password: "{{ matrix_synapse_macaroon_secret_key | pas
 
 matrix_etherpad_enabled: false
 
+matrix_etherpad_container_http_host_bind_port: "{{ '' if matrix_nginx_proxy_enabled else '127.0.0.1:9001' }}"
+
 matrix_etherpad_systemd_required_services_list: |
   {{
     ['docker.service']