UPSTREAM: 56503: MustRunAsNonRoot should reject a pod if it has non-numeric USER

[php-coder]

Cherry pick of #17512 into release-3.7 branch.

PTAL @liggitt @simo5

[openshift-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: php-coder
We suggest the following additional approver: derekwaynecarr

Assign the PR to them by writing /assign @derekwaynecarr in a comment when ready.

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[php-coder]

/assign @derekwaynecarr

[php-coder]

/retest

[php-coder]

Test flake #17682
/test extended_conformance_gce

[openshift-ci-robot]

@php-coder: The following test failed, say /retest to rerun them all:

Test name	Commit	Details	Rerun command
ci/openshift-jenkins/extended_conformance_gce	9720a2e	link	/test extended_conformance_gce

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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