Refactor nginx configuration and update README

- Moved BACKEND_URL configuration to the frontend service in the README.
- Added validation for the generated nginx configuration in the entrypoint script.
- Removed fallback nginx configuration copy from the Dockerfile.
- Adjusted nginx template to ensure proper header formatting.

This improves the deployment process and clarifies configuration instructions.

frontend/nginx.conf.template

@@ -27,7 +27,7 @@ http {
             proxy_pass http://${BACKEND_URL}/;
             proxy_http_version 1.1;
             proxy_set_header Upgrade $http_upgrade;
-            proxy_set_header Connection 'upgrade';
+            proxy_set_header Connection "upgrade";
             proxy_set_header Host $host;
             proxy_cache_bypass $http_upgrade;
             proxy_set_header X-Real-IP $remote_addr;
@@ -71,5 +71,4 @@ http {
             add_header Cache-Control "public, immutable";
         }
     }
-}
-
+}