Containerization has become a dominant paradigm in cloud-native software delivery, with orchestration platforms such as Kubernetes managing workloads that share a common host kernel across heterogeneous environments. This shared-kernel architecture exposes a critical gap: existing runtime security tools differ substantially in when and where enforcement decisions are made, in the resource types they govern, and in the policy models they support, yet no systematic framework exists for comparing these mechanisms against formal security correctness criteria. Prior surveys address container security broadly or catalog vulnerabilities, but none evaluates enforcement mechanisms against the reference monitor requirements of complete mediation, tamper resistance, and verifiability. This paper presents (i) a five-axis multi-dimensional classification framework applied to 15 representative systems that identifies five natural enforcement clusters, and (ii) a systematic evaluation of each cluster against reference monitor requirements and mandatory access control principles. The evaluation finds that synchronous kernel-level enforcement via LSM and BPF-LSM hooks achieves the highest complete mediation and tamper resistance; that verifiability remains universally immature across all clusters; and that a layered combination of syscall surface reduction, synchronous kernel enforcement, and observability-oriented detection constitutes the most robust production deployment pattern, with six open research challenges identified as prerequisites for broad adoption.