go back to the top level of this discussion and ctrl-f for "runc" then it becomes more clear. Docker is more than we usually think it is, because it makes all the stuff around containers so easy to use.
Agree. I'm working through the rubber docker lab now. The fact that docker (a) handles all the details of cgroups, overlays, namespaces; (b) is super easy to use; and (c) runs cross-platform is impressive.