docker for Mac対象でディスク削減 ref: http://qiita.com/Jey/items/0157e689fb1f428a6e6d

docker-disk-optimize.sh

#!/bin/bash
echo y | docker system prune
cd ~/Library/Containers/com.docker.docker/Data/com.docker.driver.amd64-linux || exit
mv Docker.qcow2 Docker.qcow2.org
qemu-img convert -O qcow2 Docker.qcow2.org Docker.qcow2
ls -lh Docker.qcow2 Docker.qcow2.org
rm Docker.qcow2.org

結果

./docker-disk-optimize.sh

WARNING! This will remove:
        - all stopped containers
        - all volumes not used by at least one container
        - all networks not used by at least one container
        - all dangling images
Are you sure you want to continue? [y/N] Total reclaimed space: 4.21 GB

-rw-r--r--  1 t00114  CATK\Domain Users   7.9G  2 27 13:45 Docker.qcow2
-rw-r--r--  1 t00114  CATK\Domain Users   16.2G  2 27 13:33 Docker.qcow2.org