How to categorize this issue?

/area security quality
/kind enhancement

What would you like to be added:
For a lot of Gardener components we have already minimized the container images (example). For most of them we also use non-root users to run them. I propose that we enhance the securityContext of pods with the runAsNonRoot field (read more here). This will force the kubelet to validate that the containers in a pod are started with a non-root user.

Why is this needed:
This change will prevent accidentally changing the user of a container and will help validate that containers are started as we intent.