If your argument is that it is too easy to share then you are going to have a hard time winning over users.

But honestly how hard do you think it is to integrate secure builds into a docker system? I would just stand up a private docker repository and lock the build system to that, problem solved. Or docker could roll out an update to leverage the existing namespaces and combine that with a user controlled whitelist of public key & namespace pairs. The reason docker has enjoyed this much success is because they understand sharing is #1.

Thanks for sharing, I've seen appc mentioned in a few comments, I'll check it out properly tomorrow. You also raise a good comment about blindly trusting containers which, despite having concerns about this in the past, regrettably I didn't touch on.