I installed docker on a Debian 7 machine in the following way
$ echo deb http://get.docker.io/ubuntu docker main > /etc/apt/sources.list.d/docker.list
$
As already mentioned,
docker system prune
helps, but with Docker 17.06.1 and later without pruning unused volumes. Since Docker 17.06.1, the following command prunes volumes, too:
docker system prune --volumes
From the Docker documentation: https://docs.docker.com/config/pruning/
The docker system prune command is a shortcut that prunes images, containers, and networks. In Docker 17.06.0 and earlier, volumes are also pruned. In Docker 17.06.1 and higher, you must specify the --volumes flag for docker system prune to prune volumes.
If you want to prune volumes and keep images and containers:
docker volume prune