The last time I used Docker on my Mac, it ran a VirtualBox, and a headless Ubuntu (using 2GB RAM) inside that VirtualBox. I remember creating 2 containers, and although Docker did not create another VM for the 2nd container as far as I can tell, it did create a VM with the creation of the first container.
Docker official website says Docker containers share the same kernel with the other containers. It does not say if if is the host kernel or not. From what I can tell, that means if the host OS is not, say, Ubuntu, Docker will create a guest OS. I may be too far off base, since I do not use Docker daily.
After a lot of research, I found out that Docker is similar to KVM in that it uses the host kernel (in Docker’s case, if possible as per my Ubuntu example above).
Long story short, if you are using a supported Linux flavor, you are safe (near-kernel speeds). Otherwise, it will run a Linux distro (Ubuntu I suppose) for you.