Garden makes use of runc (an Open Container Initiative project with a lot of contributions from Docker), in the same way containerd (component included in Docker) makes use of runc to run images. You won't find the docker engine being used by Garden.
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.
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.