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

.gitlab-ci.yml

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