Docker integration tests don't run with "Docker for Mac"

The docker-integration-tests can be run on Mac OS X with Docker Toolbox. However Docker Toolbox comes with a cumbersome setup (VirtualBox VM running a boot2docker Linux distro, tricky to do port mapping, ...).
A much preferable way to work with Docker on Mac OS X it is to use the Docker for Mac native application but Spark's docker-integration-tests don't run with Docker for Mac. The work-around to set DOCKER_HOST=unix:///var/run/docker.sock is not the most straight forward find.

We should upgrade the spotify docker-client dependency from 3.6.6 to 4.0.8 or later which supports "Docker for Mac" out of the box.