gitlab-ci: fix setup of debian:9 container

Debian:9 (stretch) is archived. We need to patch the sources.list
for it to be usable.

Although it's end of life, we are still interested, whether we
are able to build with such old compiler. Fix the test.

.gitlab-ci.yml

@@ -59,9 +59,9 @@ variables:
   # ".default_tag".
   ALPINE_TAG:  'tag-d53edbf2ebdd'
   CENTOS_TAG:  'tag-c374af7add25'
-  DEBIAN_TAG:  'tag-2ded62f40aeb'
+  DEBIAN_TAG:  'tag-33f59276e980'
   FEDORA_TAG:  'tag-c374af7add25'
-  UBUNTU_TAG:  'tag-2ded62f40aeb'
+  UBUNTU_TAG:  'tag-33f59276e980'
 
   ALPINE_EXEC: 'bash .gitlab-ci/alpine-install.sh'
   CENTOS_EXEC: 'bash .gitlab-ci/fedora-install.sh'

.gitlab-ci/debian-install.sh

@@ -4,6 +4,15 @@ set -ex
 
 grep -q '^VERSION=.\(9 (stretch)\|18.04.[0-9]\+ LTS\)' /etc/os-release && IS_DEBIAN_9=1 || IS_DEBIAN_9=0
 
+if [ $IS_DEBIAN_9 = 1 ]; then
+    cat > /etc/apt/sources.list <<EOF
+deb http://archive.debian.org/debian/ stretch main non-free contrib
+deb-src http://archive.debian.org/debian/ stretch main non-free contrib
+deb http://archive.debian.org/debian-security/ stretch/updates main non-free contrib
+deb-src http://archive.debian.org/debian-security/ stretch/updates main non-free contrib
+EOF
+fi
+
 if [ $IS_DEBIAN_9 = 1 ]; then
     # pam is hosted on this release to the point chfn doesn't work.
     # It's okay on Ubuntu 16.04 and 20.04 though, so keep this version specific.