Use alpine instead of busybox for cell subnet route injection

pic1 ships alpine but not busybox; ensure_cell_subnet_routes() now uses
the alpine image so route injection works on all cells.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

api/firewall_manager.py

@@ -649,7 +649,7 @@ def ensure_cell_subnet_routes(cell_links: List[Dict[str, Any]]) -> None:
                 ['docker', 'run', '--rm',
                  '--network', 'host',
                  '--cap-add', 'NET_ADMIN',
-                 'busybox',
+                 'alpine',
                  'ip', 'route', 'replace', subnet, 'via', WG_BRIDGE_IP],
                 check=False
             )