update next-build port (#17097)

Author: tjheffner

.tugboat/vhost-next.conf

@@ -29,8 +29,8 @@
     # Ignore redirecting error doc directory
     ProxyPass /.next/ !
     # Redirect requests to the running node server from next-build
-    ProxyPass / http://localhost:3000/
-    ProxyPassReverse / http://localhost:3000/
+    ProxyPass / http://localhost:3999/
+    ProxyPassReverse / http://localhost:3999/
 
     # Available loglevels: trace8, ..., trace1, debug, info, notice, warn,
     # error, crit, alert, emerg.