build-stretch-bpo does not work (when build-deps are needed from stretch-bpo)

Buildins for stretch-bpo does not work when build-dependencies are needed from stretch-backports. apt-get refuses to install them: https://salsa.debian.org/hartge-guest/bacula/-/jobs/91812

Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 builddeps:. : Depends: debhelper (>= 11.3.2~bpo9+2) but 10.2.5 is to be installed
E: Unable to correct problems, you have held broken packages.

To work around this, I forked and changed your Docker images and using mk-build-deps, aptitude and code from pbuilder-satisfydepends I hacked my way to a "solution":

https://salsa.debian.org/hartge-guest/images/blob/master/scripts/docker-build.sh

https://salsa.debian.org/hartge-guest/images/blob/master/scripts/install-build-deps.sh

https://salsa.debian.org/hartge-guest/images/blob/master/dockerfiles/dockerbuilder

This builds the packages correctly, proof here: https://salsa.debian.org/hartge-guest/bacula/-/jobs/91715

But it installs a lot more packages, making the resulting build-root a lot uncleaner than before.

A cleaner solution would be to fully adapt the code from the pbuilder build-dependency resolver.