Apply the `latest` tag in a more consistent way

Recently it happened that two pipelines got started on the same branch at nearly the same time: this led to the latest tags being intermixed, some coming from the first pipeline and some coming from the second.

This definitely confused me at some point, and can be much more confusing for people who are less used to the fun and games of containers and our CI pipelines.

With this MR we should be able to guarantee that all the latest tags come from a single pipeline.

.gitlab-ci.yml

@@ -12,6 +12,15 @@ variables:
   STABLE: auto
   TEST_TAG: test-${CI_PIPELINE_ID}
 
+default:
+  # we want to ensure that the jobs updating the `latest` tags always reflect
+  # the actual latest pipeline. when two pipelines are started at the same time
+  # on the same branch sometimes the earlier one can complete last due to
+  # scheduling or any other reason, thus breaking the previous assumption.
+  # to avoid that, make the whole pipeline interruptible up to the tagging jobs
+  # so that earlier pipelines get killed when a new one is started
+  interruptible: true
+
 before_script:
   - |
     if [ "${CI_COMMIT_BRANCH%%/*}" = "$DISTRO" ]
@@ -169,56 +178,41 @@ test-flatdeb-builder:
 publish-base-docker-image:
   extends: .publish-docker-image
   stage: publish images
-  needs:
-     - build-base-docker-image
   variables:
     image: base
 
 publish-package-builder-docker-image:
   extends: .publish-docker-image
   stage: publish images
-  needs:
-     - build-package-builder-docker-image
   variables:
     image: package-builder
 
 publish-package-source-builder-docker-image:
   extends: .publish-docker-image
   stage: publish images
-  needs:
-     - build-package-source-builder-docker-image
   variables:
     image: package-source-builder
 
 publish-documentation-builder-docker-image:
   extends: .publish-docker-image
   stage: publish images
-  needs:
-     - build-documentation-builder-docker-image
   variables:
     image: documentation-builder
 
 publish-testcases-builder-docker-image:
   extends: .publish-docker-image
   stage: publish images
-  needs:
-     - build-testcases-builder-docker-image
   variables:
     image: testcases-builder
 
 publish-image-builder-docker-image:
   extends: .publish-docker-image
   stage: publish images
-  needs:
-     - test-image-builder-shared-runner
-     - test-image-builder-kvm-runner
   variables:
     image: image-builder
 
 publish-flatdeb-builder-docker-image:
   extends: .publish-docker-image
   stage: publish images
-  needs:
-     - test-flatdeb-builder
   variables:
     image: flatdeb-builder