This relates to the containerd.io/namespace.shareable label that was added in #5043

Tracking the follow-up tasks as mentioned in #5043 (review)

We can move this along, but I still think this feature needs more work...

• I think enabling this should likely have an explicit policy
• Update of content should "copy up", not alter the shared version
• List operation should handle include the shared content, otherwise there is potential to confuse clients which use both list and stat operations to get a view of available content/images
• Snapshots, if we dare, maybe committed only