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/docker-entrypoint.sh

@@ -10,6 +10,8 @@ echo "Configuring nginx with BACKEND_URL: ${BACKEND_URL}"
 # Only substitute BACKEND_URL, preserve nginx variables like $http_upgrade
 envsubst '${BACKEND_URL}' < /etc/nginx/nginx.conf.template > /etc/nginx/nginx.conf
 
-# Execute the main command (nginx)
-exec "$@"
+# Validate the generated nginx configuration before starting
+nginx -t -c /etc/nginx/nginx.conf
 
+# Execute the main command (nginx)
+exec "$@"