[FEATURE_REQUEST] Explicit check for `securityContext.allowPrivilegeEscalation=false` on containers

[AlanMasciangelo]

Description of the problem/feature request
Add a check or option to check if containers explicitly set allowPrivilegeEscalation to false. Explicitly setting this to false is generally regarded as good security practice.

Description of the existing behavior vs. expected behavior

The existing privilege-escalation-container doesn't trigger on this condition, I imagine intentionally since allowPrivilegeEscalation is determined by a variety of factors and can be restricted in other ways.

[janisz]

@AlanMasciangelo - CEL expression solution using the new template:

• Add CEL template #1012

customChecks:
  - name: explicit-privilege-escalation-false
    template: cel-expression
    params:
      check: |
        has(object.spec.containers) && 
        object.spec.containers.exists(c, 
          !has(c.securityContext.allowPrivilegeEscalation) || 
          c.securityContext.allowPrivilegeEscalation != false) ?
        "Container should explicitly set allowPrivilegeEscalation: false" : ""
    scope:
      objectKinds: [DeploymentLike]

This checks that all containers explicitly set allowPrivilegeEscalation to false.