Backport CI changes

See merge request !2615 (merged), !2618 (merged), !2605 (merged), !2619 (merged)

.gitlab-ci/default.yml

@@ -12,16 +12,16 @@ workflow:
 
 variables:
   TRIGGER_SOURCE: "unknown"
+  MR_TARGET_BRANCH_NAME: "master"
 
 select tests:
   stage: configure
-  image: $IMAGE_REGISTRY_URL/full:dune-2.7-gcc-ubuntu-20.04
   script:
     - |
       if [[ "$TRIGGER_SOURCE" == "merge_request_event" ]]; then
           dunecontrol --opts=$DUNE_OPTS_FILE --current all
           pushd build-cmake
-            python3 ../bin/testing/findtests.py -f ../affectedtests.json -t origin/master
+            python3 ../bin/testing/findtests.py -f ../affectedtests.json -t origin/$MR_TARGET_BRANCH_NAME
           popd
       else
           echo "Skipping test selection, build/test stages will consider all tests!"