I left the cluster management group over three years ago, so I've not had much chance to work on / think about containers since then.

This code grew symbiotically with Google's kernel patches (big chunks of which were open-sourced into cgroups) and the user-space stack (which was tightly coupled with Google's cluster requirements). So open-sourcing it wouldn't necessarily have been useful for anyone. It looks like someone's done a lot of work to make this more generically-applicable before releasing it.