I thought this was going to be about the warm glow you get when you realise that the hot new framework/library/methodology you haven't got round to investigating yet, is already on the way out.
user_request -> kubernetes api -> docker api on a host
You can drop in other runtimes, like using containerd directly.
You should check out what docker and kubernetes actually are if you'd like to discuss them. Kubernetes is a management engine that controls docker containers, replacing the docker developed solution.