Kind of odd to see the homepage show Vagrant as the install mechanism, even though it supports Docker as well. In 2017, I'd think more developers are likely to run Docker than Vagrant workloads on their machine.
Those containers can run the Docker daemon though, if you like.
Edit: I _think_ what you're likely seeing (if you are seeing docker in the top output) is Concourse using the docker-image-resource to pull images for your tasks to a local docker registry.
Archlinux for example does not compile in user namespaces. Vagrant is the most cross-platform option for quick local concourse testing.