Quickstart step 3: (macOS) Start Docker
Option A: Docker for Mac
This is the easy way - just launch Docker.app and wait for it to finish loading.
Option B: docker-machine
The Docker daemon should be running in the background, which you can test by
running docker ps. If you're on OS X and using docker-machine instead of
Docker for Mac, you may have to run:
docker-machine start default eval $(docker-machine env)