Document the __meta_kubernetes_pod_container_id meta label

Signed-off-by: Ben Whetstone <ben.whetstone@sysdig.com>

docs/configuration/configuration.md

@@ -1856,6 +1856,7 @@ Available meta labels:
 * `__meta_kubernetes_pod_annotationpresent_<annotationname>`: `true` for each annotation from the pod object.
 * `__meta_kubernetes_pod_container_init`: `true` if the container is an [InitContainer](https://kubernetes.io/docs/concepts/workloads/pods/init-containers/)
 * `__meta_kubernetes_pod_container_name`: Name of the container the target address points to.
+* `__meta_kubernetes_pod_container_id`: ID of the container the target address points to. The ID is in the form `<type>://<container_id>`.
 * `__meta_kubernetes_pod_container_image`: The image the container is using.
 * `__meta_kubernetes_pod_container_port_name`: Name of the container port.
 * `__meta_kubernetes_pod_container_port_number`: Number of the container port.