Doesn't work for kube-system

Hi,

I have tried out the controller and it works as documented **except** when running pod in `kube-system`; unlike (any) other namespaces; digest is not resolved and appended when running pod in `kube-system`. Upon set `DEBUG` to true I noticed that after `received request` and `parsed resource` there is no follow-up `creating k8s keychain` and `resolved tag to digest` logs whenever pod is created in `kube-system`. I have tried bind cluster admin role to the controller SA to see if this is permission issue; but the outcome is the same.

Is it expected that k8s-digester does not work for kube-system?

Thanks!