By that logic, processes are arguably virtualisation too. They do after all use virtual memory.

Threads, processes and containers exist on a continuum.

The key is that "containers" don't actually exist -- they're just processes running under a variety of different namespaces.

It's true that Docker isn't a first-class abstraction at the level of the Linux kernel, but BSD has jails, and Solaris has Zones. This is important in some respects, but I don't see that it informs things here. Containers are still 'a thing' regardless of how they're implemented.

Curious to learn more about how jails + zones are implemented. In Linux land, I find the notion that containers are a coherent abstraction really hinders developers from understanding how their application is deployed.

Indeed they are! The notion that each process has its separate address space is called virtual memory for that reason.

See also cgroups: while this feature is used by the container run times, it predates Docker, and can be used standalone with normal processes.

Indeed. I made a website for testing npm packages inside a cgroup/unshared "container" - about 6 months before docker came out.

If only I had realised that could have been useful for more than testing npm packages...