While playing around with Docker and orchestration (kubernetes) I had to install and use minikube to create a simple sandbox environment. At the beginning I thought that minikube installs some kind of VM and run the "minified" kubernetes environment inside the same, however, after the installation listing my local Docker running containers I found minikube running as a container!!
Why minikube itself run as a Docker container? and how can it runs other containers?
Experimental Docker support looks to have been added in minikube 1.7.0, and started becoming the default runtime in minikube 1.9.0. As I'm writing this, current is 1.15.1.
The minikube documentation on the "docker" driver notes, particularly on a native-Linux host, there is not an intermediate virtual machine: if you can run Kubernetes in a container, it can use the entire host system's resources without special configuration or partitioning. The previous minikube-on-VirtualBox installation required preallocating memory and disk to the VM, and it was easy to get those settings wrong. Even on non-Linux hosts, if you're running Docker Desktop, sharing its hidden Linux VM can improve resource utilization, and you don't need to decide to allocate exactly 2 GB RAM to Docker Desktop and exactly 4 GB to the minikube VM.
For a long time it's been possible, but discouraged, to run a separate Docker daemon inside a Docker container; similarly, it's possible, but usually discouraged, to run a multi-process init system in a container. If you do both of these things then you can have the core Kubernetes components (etcd, apiserver, kubelet, ...) inside a single container pretending to be a Kubernetes node. It also helps here that Kubernetes already knows how to pull Docker images, which minimizes some of the confusing issues with running Docker in Docker.