Rewrite build_debian_packages

Rewrite this in python so that it can be run in parallel.

docker/build_debian_packages.sh

@@ -1,46 +0,0 @@
-#!/bin/bash
-
-# Build the Debian packages using Docker images.
-#
-# This script builds the Docker images and then executes them sequentially, each
-# one building a Debian package for the targeted operating system. It is
-# designed to be a "single command" to produce all the images.
-#
-# By default, builds for all known distributions, but a list of distributions
-# can be passed on the commandline for debugging.
-
-set -ex
-
-cd `dirname $0`
-
-if [ $# -lt 1 ]; then
-    DISTS=(
-        debian:stretch
-        debian:buster
-        debian:sid
-        ubuntu:xenial
-        ubuntu:bionic
-        ubuntu:cosmic
-    )
-else
-    DISTS=("$@")
-fi
-
-# Make the dir where the debs will live.
-#
-# Note that we deliberately put this outside the source tree, otherwise we tend
-# to get source packages which are full of debs. (We could hack around that
-# with more magic in the build_debian.sh script, but that doesn't solve the
-# problem for natively-run dpkg-buildpakage).
-
-mkdir -p ../../debs
-
-# Build each OS image;
-for i in "${DISTS[@]}"; do
-    TAG=$(echo ${i} | cut -d ":" -f 2)
-    docker build --tag dh-venv-builder:${TAG} --build-arg distro=${i} -f Dockerfile-dhvirtualenv .
-    docker run -it --rm --volume=$(pwd)/../\:/synapse/source:ro --volume=$(pwd)/../../debs:/debs \
-           -e TARGET_USERID=$(id -u) \
-           -e TARGET_GROUPID=$(id -g) \
-           dh-venv-builder:${TAG}
-done