with docker, your guest machines run the same kernel as your host, but a different root filesystem and init system. the result is that you can run centOS on docker on Ubuntu, but you'll get centOS with the Ubuntu kernel. So, you lose some flexibility with the kernel of your VM. But a docker container is extremely light weight. it boots in a few hundred milliseconds, and it only costs a small amount more resources to run a service inside docker than if you had just run the service on the host system directly.