[gtkmm/kjellahl/testci] CI: Switch to debian:testing

[Kjell Ahlstedt <kjellahl src gnome org>]

commit d344a0fe2becf37af288f50ff5072978c56922ab
Author: Kjell Ahlstedt <kjellahlstedt gmail com>
Date:   Mon Sep 13 16:50:31 2021 +0200

    CI: Switch to debian:testing

 .gitlab-ci.yml | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)
---
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 7de0949c..89c38df2 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -1,4 +1,4 @@
-image: ubuntu:devel
+image: debian:testing
 
 stages:
   - deps
@@ -28,7 +28,9 @@ variables:
 .build_default:
   before_script:
     - export DEBIAN_FRONTEND=noninteractive
-    - apt update && apt -y upgrade && apt -y install $DEPENDENCIES
+    - apt update
+    - apt -y upgrade
+    - apt -y install $DEPENDENCIES
 
 # Clear the cache manually.
 clear_cache: