The standard data directory used by docker is /var/lib/docker, and since this directory will store all your images, volumes, etc. it can become quite large in a relative small amount of time.
If you want to move the docker data directory on another location you can follow the following simple steps.
1. Stop the docker daemon
sudo service docker stop
2. Add a configuration file to tell the docker daemon what is the location of the data directory
Create docker daemon configuration /etc/docker/daemon.json with following content: