change the default docker root dir following this steps( docker version: 18.0)
[I want to change its default /var/lib/docker to /data/docker]
0. $> stop docker service: service docker stop
1. $> vi /usr/lib/systemd/system/docker.service
2. edit the ExecStart item:$> ExecStart=/usr/bin/dockerd -D --data-root /data/docker (-D is debug mode)
3. reload daemon: $> systemctl daemon-reload
4. start docker: $> service docker start
5. check it work or not: $> docker info