Update contrib/mkimage/debootstrap to remove /var/lib/apt/lists

(trimming at least 40MB and forcing "apt-get update" in dependent images before packages can be installed)

Closes #6217

The `rm` is safe since we've been careful about our use of `$rootfsDir` (which, if isn't set properly, this whole script will cause all kinds of fun damage, so it behooves us to make sure `mkimage.sh`, which is the only way to correctly invoke this script, correctly passes the proper rootfs directory).