Automatic merge from submit-queue (batch tested with PRs 56589, 56503). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.
MustRunAsNonRoot should reject a pod if it has non-numeric USER
**What this PR does / why we need it**:
This PR modifies kubelet behavior to reject pods with non-numeric USER instead of showing a warning.
**Special notes for your reviewer**:
Related discussion: https://github.com/kubernetes/community/pull/756#discussion_r143694443
**Release note**:
```release-note
kubelet: fix bug where `runAsUser: MustRunAsNonRoot` strategy didn't reject a pod with a non-numeric `USER`.
```
PTAL @pweil- @tallclair @liggitt @Random-Liu
CC @simo5 @adelton