Cleaning up apt cache to optimize disk space

I tested it and got this :
before : 793.5 MB
after : 784.1 MB
All this 9.4MB is unnecessary files, as recommended in Debian mkimage :
"delete all the apt list files since they're big and get stale quickly
this forces "apt-get update" in dependent images, which is also good"
https://github.com/docker/docker/blob/master/contrib/mkimage/debootstrap