Add "Acquire::GzipIndexes" to contrib/mkimage/debootstrap

This results in a dramatic improvement in the size of individual "apt-get update" layers (on a clean wheezy base, from "29.88 MB" down to "8.273 MB").

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)
Merge pull request #7064 from tianon/debootstrap-gzipindexes

Add "Acquire::GzipIndexes" to contrib/mkimage/debootstrap