Create pbuilder base.tgz with debootstrap from stretch

buster's version doesn't work inside docker, see:
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=930684

Note that apt cache is just disabled for building the base.tgz
as stretch debootstrap doesn't support it. Normal package
builds will still use apt caching.

Change-Id: I1e922833c94b19888af57a76627f11192b02f830
2 files changed