[add] added support for custom nginx location config files

files/upstream_configs/upstream.conf.example

@@ -0,0 +1,13 @@
+        location / {
+            # allow for upstream to be offline by setting
+            # docker dns resolver and a variable for the server
+            resolver           127.0.0.11 valid=30s;
+            proxy_pass         http://$upstream;
+
+            proxy_redirect     off;
+            proxy_set_header   Host $host;
+            proxy_set_header   X-Real-IP $remote_addr;
+            proxy_set_header   X-Forwarded-For $proxy_add_x_forwarded_for;
+            proxy_set_header   X-Forwarded-Host $server_name;
+            proxy_set_header   X-Forwarded-Upstream $upstream;
+        }