Dockerfile: add LABEL for podman auto-update

podman auto-update pulls down new container images and restarts containers configured for auto updates. To make use of auto updates, the container or Kubernetes workloads must run inside a systemd unit. After a successful update of an image, the containers using the image get updated by restarting the systemd units they run in.

To configure a container for auto updates, it must be created with the io.containers.autoupdate label:

If the label is present and set to registry, Podman reaches out to the corresponding registry to check if the image has been updated. The label image is an alternative to registry maintained for backwards compatibility. An image is considered updated if the digest in the local storage is different than the one of the remote image. If an image must be updated, Podman pulls it down and restarts the systemd unit executing the container.