Use modern CI script

This introduces the same CI script also used in the other agnumpde repositories.

.gitlab-ci.yml

 ---
 # Install external dependencies
 
-dune:git--clang:
-  image: duneci/dune:git
-  script: duneci-standard-test --opts=/duneci/opts.clang
+dune:git  clang  C++17:
+  image: registry.dune-project.org/docker/ci/dune:git-debian-10-clang-7-libcpp-17
+  script: duneci-standard-test
+
+dune:git  gcc-8  C++17:
+  image: registry.dune-project.org/docker/ci/dune:git-debian-10-gcc-8-17
+  script: duneci-standard-test
 
-dune:git--gcc:
-  image: duneci/dune:git
+dune:git  gcc-6  C++14:
+  image: registry.dune-project.org/docker/ci/dune:git-debian-9-gcc-6-14
   script: duneci-standard-test