> container is an abstract idea

In this context, it's not, it's specifically referring to a process running in a cgroup.

> LXC is one way to do so, runc is another way to do so, docker is a third way to do so

Docker is a suite of tools for managing running LXC or runc, both of which set up processes running under cgroups.

> Now if you took some other OS, there'd be different solutions, each with slightly different details and thus properties, but same idea

You should read the article.