@h This really nicely illustrates why Plan 9 is so superior to Linux here. Plan 9 is basically containerized out of the box; it's impossible to run anything *not* in a container. It's intrinsic in how Plan 9 works.
Here, we see how Linux tries to do the same, largely succeeding, but you have to be really persnickety about the fine details.
Docker would never succeed on Plan 9; there's no market for it. On Linux, it is a band-aid on top of the worse-is-better approach.