docker file directory migration

• stop service docker stop
• Backup data directory /var/lib/docker
• Migrate the /var/lib/docker directory to the /mnt/ directory: mv /var/lib/docker /mnt/docker
• 建 symlink: ln -s /mnt/docker /var/lib/docker
• Make sure the folder type is symlink type ls /var/lib/docker
• Start the service docker stop .