In addition to the other answers, I had the same issue using Docker for Mac but docker ps -a did not show any running containers. Running docker images shows:
docker/desktop-storage-provisioner v1.0 605a0f683b7b 2 months ago 33.1MB
k8s.gcr.io/kube-controller-manager v1.15.5 1399a72fa1a9 6 months ago 159MB
k8s.gcr.io/kube-proxy v1.15.5 cbd7f21fec99 6 months ago 82.4MB
k8s.gcr.io/kube-apiserver v1.15.5 e534b1952a0d 6 months ago 207MB
k8s.gcr.io/kube-scheduler v1.15.5 fab2dded59dd 6 months ago 81.1MB
docker/kube-compose-controller v0.4.23 a8c3d87a58e7 11 months ago 35.3MB
docker/kube-compose-api-server v0.4.23 f3591b2cb223 11 months ago 49.9MB
k8s.gcr.io/coredns 1.3.1 eb516548c180 16 months ago 40.3MB
k8s.gcr.io/etcd 3.3.10 2c4adeb21b4f 17 months ago 258MB
k8s.gcr.io/pause 3.1 da86e6ba6ca1 2 years ago 742kB
All of these images are used by the Docker app if you have Kubernetes enabled. In the Docker for Mac preferences, there is a section for Kubernetes. If you click "Show system containers (advanced)", they will appear in docker ps.
If you wish to delete these images, you need to disable Kubernetes via the Docker for Mac app, then try again.