The Docker machine is running. But you need to export some environment to connect to the Docker machine. By default, the
docker CLI client is trying to communicate to the daemon using
http+unix://var/run/docker.sock (as shown in the error message).
Export the correct environment variables using
eval $(docker-machine env dev) and then try again. You can also just run
docker-machine env dev to see the environment variables it will export. Notice that one of them is
DOCKER_HOST, just as the error message suggests you may need to set.