feat(deploy): update Nginx configuration for Authentik proxy

- Added resolver and upstream variable for Authentik server in Nginx configuration.
- Enhanced proxy_pass directive to use the defined upstream variable for better maintainability.

deploy/nginx/default.conf.template

@@ -98,7 +98,9 @@ server {
     }
 
     location /auth/ {
-        proxy_pass http://authentik-server:9000;
+        resolver 127.0.0.11 valid=10s ipv6=off;
+        set $authentik_upstream authentik-server:9000;
+        proxy_pass http://$authentik_upstream;
         proxy_http_version 1.1;
         proxy_set_header Host $host;
         proxy_set_header X-Real-IP $remote_addr;