Revert "Bump CI image version from Debian 8 to Debian 9 (stretch)"

This reverts commit 41c62621d6 to get
the pipeline green again.  It looks like the broken pipeline caused
the website pipeline to fail on downloading the frontends artifact.
Not sure why it hasn't failed earlier.

.gitlab-ci.yml

@@ -1,5 +1,5 @@
 # .gitlab-ci.yml -- to build source tarball snapshots
-# Copyright (C) 2017, 2019  Olaf Meeuwissen
+# Copyright (C) 2017  Olaf Meeuwissen
 #
 # License: GPL-3.0+
 
@@ -11,7 +11,7 @@ stages:
 
 archive:
   stage: archive
-  image: $REGISTRY_HUB:debian-stretch-full
+  image: $REGISTRY_HUB:debian-8-full
   before_script:
   - apt-get update --quiet
   - apt-get install --quiet --assume-yes libsane-dev