fix: prevent duplicate plugin installation to avoid infinite loop

- Check if plugin is already installed before attempting installation
- Prevents repeated installation attempts in Docker environments
- Reduces startup time and avoids potential infinite loops

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: Leechael

custom-domain/dstack-ingress/scripts/certman.py

@@ -30,6 +30,15 @@ def install_plugin(self) -> bool:
             print(f"No certbot package defined for {self.provider_type}")
             return False
 
+        # Check if plugin is already installed
+        try:
+            if self.provider.CERTBOT_PLUGIN == "dns-namecheap":
+                import certbot_dns_namecheap.dns_namecheap
+                print(f"Plugin {self.provider.CERTBOT_PACKAGE} is already installed")
+                return True
+        except ImportError:
+            pass  # Plugin not installed, continue with installation
+
         print(f"Installing certbot plugin: {self.provider.CERTBOT_PACKAGE}")
 
         # Use virtual environment pip if available