The current method of using "apt-get build-dep -y ." does only work for simple dependencies, but not for experimental or *-backports.
For those use cases, apt-get is not smart enough to satisfy the build-dependencies from additional repositories, even if requested via versioned build-dependency.
sbuild and pbuilder instead use aptitude and a dummy package to pull in the build-depdencies with some additional trickery to convince the resolver to do the Right Thing.
This commit implements this variant for docker-builder, fixing pipeline#21 (closed) and bringing the way builds are done more in line with how the other package building tools in Debian work.
Doing the builds with this modification has been tested in the bacula-team repository for the last 4 weeks with successful builds for Unstable, stretch-backports and Buster.