I think that Plan 9's filesystem-based namespacing (and lack of a superuser) actually have a lot to offer for container-like solutions. Any Plan 9 user can set up namespacing of the network and of resources and spawn a process within that restricted namespace.

The whole process is much simpler, I think, than that of creating a Linux container (that's the whole reason Docker exists: to simplify & abstract something which isn't really inherently complex, but is accidentally complex).

Plan 9 certainly wasn't perfect, but it had some really high-quality ideas we still haven't assimilated in mainstream platforms.