C-WCOW: Enforce hashes on already mounted CIMs

[MahatiC]

Prior to this change, if you had two containers with the same image layers, the second container would not get captured in the SecurityPolicy metadata and hence would not enforce the policy. SecurityPolicy pkg captures container image layer hashes and associates them with the ContainerID during CIM mount operation. But because the CIMs are already mounted (for the previous container), this process would not occur for the second container with the same layers.

This change caches the image layer hashes for each container and SecurityPolicy instance will capture it in the metadata later on (during layer combining) if it wasn't already captured previously for this ContainerID and enforces policy.