add upstream url and rewrite do resolve request dynamically

partially attends to #475

data/nginx/routes/webmail.conf

@@ -1,5 +1,9 @@
 location /webmail/ {
-	proxy_pass http://webmail:8080/;
+	resolver 127.0.0.11 valid=30s;
+	set $upstream_host webmail;
+	proxy_pass http://$upstream_host:8080;
+	rewrite /webmail(.*) $1 break;
+
 	proxy_buffering off;
 	proxy_http_version 1.1;
 	proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;