Remove variable usage from proxy_pass directive to fix resolution issues

By using a static URL, the backend server can be accessed reliably, avoiding the common 404 errors or "no resolver defined" issues seen when variables are used.

backend/templates/_location.conf

@@ -11,7 +11,7 @@
     set $proxy_server         "{{ forward_host }}";
     set $proxy_port           {{ forward_port }};
 
-    proxy_pass       $proxy_forward_scheme://$proxy_server:$proxy_port{{ forward_path }};
+    proxy_pass       {{ forward_scheme }}:{{ forward_host }}:{{ forward_port }}{{ forward_path }};
 
     {% include "_access.conf" %}
     {% include "_assets.conf" %}