Fix Shellcheck SC2154: variable possibly undefined

var is referenced but not assigned.

https://github.com/koalaman/shellcheck/wiki/SC2154

Signed-off-by: Dan Callahan <danc@element.io>

docker/build_debian.sh

@@ -5,7 +5,7 @@
 set -ex
 
 # Get the codename from distro env
-DIST=`cut -d ':' -f2 <<< $distro`
+DIST=`cut -d ':' -f2 <<< ${distro:?}`
 
 # we get a read-only copy of the source: make a writeable copy
 cp -aT /synapse/source /synapse/build