cannot get emoji png with a proxy server like nginx

My nodebb is running behind nginx by using a proxy port.The default ```nconf.get('url')``` returns the original ```url:port``` which is shielded by nginx.I got a wrong url of emoji image like ```host:4567/plugins/nodebb-plugin-emoji-extended/images/1234.png```,it should be ```host:80/plugins/nodebb-plugin-emoji-extended/images/1234.png```.So I put a key-value ```"proxy_port":"80"``` in the ```nodebb/config.json``` to declare a proxy port.