Docker is a wrapper on top of kernel containers in the same way that Emacs is a wrapper on top of the kernel filesystem API. I've written a 150-line container

Docker is a wrapper on top of kernel containers in the same way that Emacs is a wrapper on top of the kernel filesystem API. I've written a 150-line container manager in shell before (because we had a use case where both Docker was not the right technical fit out of the box and the organization had barely any operational experience with even normal use of Docker), and it's great that containers are built into the kernel and you can just use unshare and a bunch of bind mounts if you'd like, but Docker is its own quite substantial product and brings its own approach to building systems (Dockerfiles, container networking, etc.) that aren't implied by or required by the kernel API.