"Container solutions of the kind that are like docker" vs "Container solutions in general of which docker is an example". If he is using docker to write-off all container solutions, then that's a mistake, since it sounds like using the underlying container solution (on which docker then adds the ability to manage system images) is exactly what he wants.
Yeah, perhaps namespaces will satisfy them, but from what I've seen network namespaces leave something to be desired. I found that it wasn't too hard to roll my own container implementation and hook it up to a tool that was already providing software environments to unprivileged users, so I think there's plenty of room for other people to take the primitives that Linux provides and run with them.
