Fix debian packages for sid being called buster. (matrix-org#5775)

* Fix debian packages for sid being called buster.

I don't know why the sid images return buster as its codename in
`lsb_release` but it does, so lets just grab the codename from the
distro we pass into dockerfile

* Newsfile

changelog.d/5775.bugfix

@@ -0,0 +1 @@
+Fix debian packaging scripts to correctly build sid packages.

docker/Dockerfile-dhvirtualenv

@@ -42,6 +42,11 @@ RUN cd dh-virtualenv-1.1 && dpkg-buildpackage -us -uc -b
 ###
 FROM ${distro}
 
+# Get the distro we want to pull from as a dynamic build variable
+# (We need to define it in each build stage)
+ARG distro=""
+ENV distro ${distro}
+
 # Install the build dependencies
 #
 # NB: keep this list in sync with the list of build-deps in debian/control

docker/build_debian.sh

@@ -4,7 +4,8 @@
 
 set -ex
 
-DIST=`lsb_release -c -s`
+# Get the codename from distro env
+DIST=`cut -d ':' -f2 <<< $distro`
 
 # we get a read-only copy of the source: make a writeable copy
 cp -aT /synapse/source /synapse/build