Thanks, you are right, there is a huge Qemu Image under ~/Library/Containers/com.docker.docker, which is not easy to work with :-/
I had already removed all containers, all images, and all volumes, but the contents of the volume kept coming back. Then I discovered that somehow the docker containers were still running, because I could communicate with them though locahost!
I fixed the problem by rebooting the machine. I suspect that the update from Docker for Mac beta8 to beta9 somehow kept old containers running. Now everything works fine.