Potential fix for code scanning alert no. 8: Clear-text storage of sensitive information

Co-authored-by: Copilot Autofix powered by AI <62310815+github-advanced-security[bot]@users.noreply.github.com>

Author: kvinwang

custom-domain/dstack-ingress/scripts/dns_providers/linode.py

@@ -44,6 +44,8 @@ def setup_certbot_credentials(self) -> bool:
 
             # Write credentials file
             with open(credentials_file, "w") as f:
+                f.write("# WARNING: This file contains sensitive credentials for Linode DNS API.\n")
+                f.write("# Ensure this file is kept secure and not shared.\n")
                 f.write(f"dns_linode_key = {self.api_token}\n")
 
             # Set secure permissions