Add a ton of simple improvements to contrib/mkimage/debootstrap, especially comments in the extra files put inside the image

This also adds support for squeeze-lts. :)

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #6572 from tianon/better-debootstrap-self-documentation

Add a ton of simple improvements to contrib/mkimage/debootstrap, especially comments in the extra files put inside the image