GitLab CI: Use `matrix` to build for multiple distros

Instead of repeating the job for multiple distros, we can use the new
`matrix` functionality to instantiate the job multiple times.

Here we use this to build the package for all supported distributions.

.gitlab-ci.yml

@@ -2,19 +2,14 @@ stages:
   - build
   - deploy
 
-build-debian-bullseye:
+build:
   stage: build
-  image: debian:bullseye
-  artifacts:
-    expire_in: 1h
-    paths:
-      - output
-  script:
-    - ./build.sh
-
-build-debian-buster:
-  stage: build
-  image: debian:buster
+  parallel:
+    matrix:
+      - IMAGE:
+        - 'debian:buster' # Debian 10 (buster)
+        - 'debian:bullseye' # Debian 11 (bullseye)
+  image: $IMAGE
   artifacts:
     expire_in: 1h
     paths: