[ci] Trigger pipeline as a child pipeline

.gitlab-ci.yml

-default:
-  image: registry.dune-project.org/docker/ci/dune:2.7-debian-11-gcc-9-20
-
-cache:
-  key: ${CI_COMMIT_REF_SLUG}
-  paths:
-    - dep_modules
-
 stages:
-  - build
-  - test
+  - trigger pipelines
 
-# rules for build & test:
-# - pipelines are built for commits to master, tags, merge requests
-# - Within merge requests, we require manual trigger of the build stage
-#   to start off the pipeline
-.build_rules:
+# rules for the default triggers:
+# - pipelines are triggered for commits to master, tags, merge requests
+# - Within merge requests, we require to start the pipeline manually
+#   by clicking play for the trigger
+.default-trigger:
+  stage: trigger pipelines
+  trigger:
+    include:
+      - local: .gitlab-ci/default.yml
+    strategy: depend
   rules:
     - if: $CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH
-    - if: $CI_PIPELINE_SOURCE=="pipeline"
+    - if: $CI_PIPELINE_SOURCE == "pipeline"
     - if: $CI_PIPELINE_SOURCE == "merge_request_event"
       when: manual
 
-.test_rules:
-  rules:
-    - if: $CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH
-    - if: $CI_PIPELINE_SOURCE=="pipeline"
-    - if: $CI_PIPELINE_SOURCE == "merge_request_event"
-
-build lecture:
-  extends: .build_rules
-  stage: build
-  before_script:
-    - mkdir -p dep_modules; cd dep_modules
-    - git clone -b releases/2.7 https://gitlab.dune-project.org/extensions/dune-foamgrid.git
-    - git clone -b releases/2.7 https://gitlab.dune-project.org/extensions/dune-alugrid.git
-    - git clone -b master https://git.iws.uni-stuttgart.de/dumux-repositories/dumux.git
-    - dunecontrol --opts=/duneci/dune.opts --only=dune-foamgrid all
-    - dunecontrol --opts=/duneci/dune.opts --only=dune-alugrid all
-    - dunecontrol --opts=/duneci/dune.opts --only=dumux all
-    - cd ..
-  script:
-    - cd ..
-    - dunecontrol --opts=/duneci/dune.opts --only=dumux-lecture all
-    - cd dumux-lecture/build-cmake
-    - make -j 8 build_tests
-  artifacts:
-    paths:
-      - build-cmake
-    expire_in: 3 hours
+debian11-gcc9-cpp20:
+  extends: .default-trigger
+  variables:
+    IMAGE: registry.dune-project.org/docker/ci/dune:2.7-debian-11-gcc-9-20
 
-test lecture:
-  extends: .test_rules
-  stage: test
-  script:
-    - cd build-cmake
-    - duneci-ctest -j4 --no-tests=error
-  needs:
-    - job: build lecture
-      artifacts: true
+ubuntu18.04-clang6-cpp17:
+  extends: .default-trigger
+  variables:
+    IMAGE: registry.dune-project.org/docker/ci/dune:2.7-ubuntu-18.04-clang-6-17

.gitlab-ci/default.yml

+default:
+  image: $IMAGE
+
+cache:
+  key: ${CI_COMMIT_REF_SLUG}-$IMAGE
+  paths:
+    - dep_modules
+
+stages:
+  - build
+  - test
+
+workflow:
+  rules:
+    - if: $CI_PIPELINE_SOURCE=="parent_pipeline"
+
+build lecture:
+  stage: build
+  before_script:
+    - mkdir -p dep_modules
+    - pushd dep_modules
+    - >
+      if [ ! -d dune-foamgrid ]; then
+        git clone -b releases/2.7 --depth 1 https://gitlab.dune-project.org/extensions/dune-foamgrid.git;
+        dunecontrol --opts=/duneci/dune.opts --only=dune-foamgrid all;
+      fi
+    - >
+      if [ ! -d dune-alugrid ]; then
+        git clone -b releases/2.7 --depth 1 https://gitlab.dune-project.org/extensions/dune-alugrid.git;
+        dunecontrol --opts=/duneci/dune.opts --make-options="-j8" --only=dune-alugrid all;
+      fi
+    - >
+      if [ ! -d dumux ]; then
+        git clone -b master --depth 1 https://git.iws.uni-stuttgart.de/dumux-repositories/dumux.git;
+        dunecontrol --opts=/duneci/dune.opts --only=dumux all
+      fi
+    - popd
+  script:
+    - cd ..
+    - dunecontrol --opts=/duneci/dune.opts --only=dumux-lecture all
+    - cd dumux-lecture/build-cmake
+    - make -k -j 8 build_tests
+  artifacts:
+    paths:
+      - build-cmake
+    expire_in: 3 hours
+
+test lecture:
+  stage: test
+  script:
+    - cd build-cmake
+    - duneci-ctest -j4 --no-tests=error
+  needs:
+    - job: build lecture
+      artifacts: true