gitlab-ci: Use PureOS byzantium

This way we get packages suitable for byzantium

.gitlab-ci.yml

@@ -5,16 +5,20 @@ stages:
  - package
  - test-package
 
-package:deb-debian-buster:
+variables:
+    L5_DOCKER_IMAGE: pureos/byzantium
+    L5_ADD_SCRATCH_CI: 'false'
+
+package:deb-debian:
   extends: .l5-build-debian-package
 
-autopkgtest-debian-buster-package:
+autopkgtest-debian-package:
   dependencies:
-    - package:deb-debian-buster
+    - package:deb-debian
   extends: .l5-autopkgtest-debian-package
 
-lintian-debian-buster-package:
+lintian-debian-package:
   dependencies:
-    - package:deb-debian-buster
+    - package:deb-debian
   extends: .l5-lintian-debian-package