Add Docker container health detector

Monitors for containers stuck in restart loops or unhealthy states:
- Critical: restart loop (≥3 restarts)
- Warning: restarting, exited abnormally, or unhealthy

Disabled by default in config.json.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

config.json

@@ -52,6 +52,16 @@
 				"HOSTS": "8.8.8.8,google.com",
 				"TIMEOUT": "5"
 			}
+		},
+		{
+			"name": "docker",
+			"enabled": false,
+			"script": "detectors/docker.py",
+			"env": {
+				"CHECK_INTERVAL": "60",
+				"RESTART_THRESHOLD": "3",
+				"CONTAINERS": ""
+			}
 		}
 	]
 }