Modernize CI script

.gitlab-ci.yml

 ---
-dune:git  clang  C++17:
-  image: registry.dune-project.org/docker/ci/dune:git-debian-10-clang-7-libcpp-17
+before_script:
+  - duneci-install-module https://gitlab.dune-project.org/extensions/dune-alugrid.git
+
+.common: &common
   script: duneci-standard-test
+  artifacts:
+    expire_in: 2 years
+    reports:
+      junit: junit/*.xml
 
 dune:git  gcc-8  C++17:
+  <<: *common
   image: registry.dune-project.org/docker/ci/dune:git-debian-10-gcc-8-17
-  script: duneci-standard-test
 
-dune:git  gcc-6  C++14:
-  image: registry.dune-project.org/docker/ci/dune:git-debian-9-gcc-6-14
-  script: duneci-standard-test
+dune:git  clang  C++17:
+  <<: *common
+  image: registry.dune-project.org/docker/ci/dune:git-debian-10-clang-7-libcpp-17