clean docker library dir

[Corey H <coreyh@free.fr>]

Hello

My VPS has docker installed, which consumes a lot of disk.

/var/lib/docker# du -sh .
71.0G	.

I tried 'docker system prune' which seems not to delete the libraries it 
intalled.

Do you know how to clean the objects in '/var/lib/docker'?

Thanks.