Update X-Forwarded-For header instead of overriding it

When this container is behind a reverse proxy itself it overrides the incoming `X-Forwarded-For` header from the downstream reverse proxy. This is problematic because Snadrop heavily relies on this information to create rooms.

root/defaults/default

@@ -23,6 +23,6 @@ server {
         proxy_pass http://localhost:3000;
         proxy_set_header Connection "upgrade";
         proxy_set_header Upgrade $http_upgrade;
-        proxy_set_header X-Forwarded-for $remote_addr;
+        proxy_set_header X-Forwarded-for $proxy_add_x_forwarded_for;
     }
 }