Host should be $proxy_host, not $host

* $host is the Host sent by the user's browser. This is the correct
  setting if the service has built-in support for being proxied.

* $proxy_host is the Host as if the browser would run on the
  proxy. This is the correct setting if the service does not have
  built-in support for reverse proxies. It is also the nginx default.

* In nginx, you cannot unset the Host header. Configuring headers
  multiple times just sends multiple values with the http request. So
  there is no way to "fix" the Host by adding a custom header if it is
  already set.

For these reasons, Host should not be set (and default to
$proxy_host). In the unlikely case that your service needs something
else you can then just set the header in the GUI.

Fixes NginxProxyManager#2675

docker/rootfs/etc/nginx/conf.d/include/proxy.conf

@@ -1,5 +1,4 @@
 add_header X-Served-By $host;
-proxy_set_header Host $host;
 proxy_set_header X-Forwarded-Scheme $scheme;
 proxy_set_header X-Forwarded-Proto $scheme;
 proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;