Draft: package-source-builder: allow cross-building packages

Allow cross-building packages by accepting a Debian architecture from an environment variable named $ARCH.

Allow build profiles to be passed to debuild by using the environment variable $DEB_BUILD_PROFILES.

First build the docker container (after cloning this branch):

docker build package-source-builder --build-arg RELEASE=v2023dev2 --tag apertis-v2023dev2-crossbuild

Cross-building the Kernel package for arm64 (after cloning the Kernel package tree):

git clone https://gitlab.apertis.org/pkg/linux.git
cd linux
pristine-lfs checkout --auto
docker run --rm -it \
  -v ${PWD}:/mnt/pkg \
  -w /mnt/pkg \
  -e DEB_BUILD_OPTIONS=parallel=$(nproc) \
  -e DEB_BUILD_PROFILES="pkg.linux.notools nodoc noudeb" \
  -e ARCH=arm64 \
  apertis-v2023dev2-crossbuild \
  ci-buildpackage