support adding an extra apt source from the build job

gitlab-ci-git-buildpackage

@@ -16,6 +16,10 @@ if [[ ! -f $lockfile ]] ; then
         apt_get="$apt_get $APT_GET_OPTIONS"
     fi
 
+    if [ ! -z "$EXTRA_APT_SOURCE" ]; then
+        printf "\n$EXTRA_APT_SOURCE\n" >> /etc/apt/sources.list
+    fi
+
     if [ ! -z "$DEBIAN_RELEASE" ]; then
         sed -i "s, testing , $DEBIAN_RELEASE ," /etc/apt/sources.list
     fi