add docker-in-docker so we can run megalinter inside a devcontainer (#36)

.devcontainer/devcontainer.json

@@ -76,6 +76,9 @@
 		},
 		"ghcr.io/devcontainers/features/python:1": {
 			"version": "latest"
+		},
+		"ghcr.io/devcontainers/features/docker-in-docker:1": {
+			"version": "latest"
 		}
 	}
 }