use $http_host in nginx proxy header

$host is the hostname, $http_host is `hostname[:port]`, which is what's needed here

$host works fine in the example because it uses the default port 80, but if it's on a different port
it will differ from the http Host header, resulting in cross-origin check errors.

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