Do not generally permit containers to be run as the root user. Rationale: Containers may run as any Linux user. Containers which run as the root user, whilst constrained by Container Runtime security features still have a escalated likelihood of container breakout. Ideally, all containers should run as a defined non-UID 0 user. There should be at least one PodSecurityPolicy (PSP) defined which does not permit root users in a container. If you need to run root containers, this should be defined in a separate PSP and you should carefully check RBAC controls to ensure that only limited service accounts and users are given permission to access that PSP.
Create a PSP as described in the Kubernetes documentation, ensuring that the .spec.runAsUser.rule is set to either MustRunAsNonRoot or MustRunAs with the range of UIDs not including 0. Impact: Pods with containers which run as the root user will not be permitted. Default Value: By default, PodSecurityPolicies are not defined. References: https://kubernetes.io/docs/concepts/policy/pod-security-policy/#enabling-pod-security-policies