Merge pull request jupyterhub#4671 from minrk/nginx-http-host

use $http_host in nginx proxy header

docs/source/howto/configuration/config-proxy.md

@@ -79,7 +79,7 @@ server {
     location / {
         proxy_pass http://127.0.0.1:8000;
         proxy_set_header X-Real-IP $remote_addr;
-        proxy_set_header Host $host;
+        proxy_set_header Host $http_host;
         proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
 
         # websocket headers