Nginx proxy config - websocket connections get 403

The reason is most likely a new cross-origin check: https://gitlab.com/gitlab-org/omnibus-gitlab/issues/1966
Fixed it by adding headers to the nginx forwarding:

 location /api/v3/users/websocket {
  proxy_pass            http://127.0.0.1:8065;
  (...)
  proxy_set_header      Host (your host name);
  proxy_set_header      X-Forwarded-For $remote_addr;
}