check for root privileges in make_chroot.sh
stop copying chroot_tasks.sh to file named chroot
install Ruby and bandshell gem, this fixes #2
check root privileges in make_bootable_image.sh

code copied for now from make_chroot.sh but probably should
think about moving common functionality into another file if
it continues to be used
set up make_chroot.sh to use apt-cacher-ng

this takes load off the Debian servers but makes apt-cacher-ng a build
prerequisite. If apt-cacher-ng is not available, its use may be disabled
simply by changing the MIRROR_URL in make_chroot.sh.