Merge branch 'patch-1' into 'master'

Update .gitlab-ci.yml for newer CI images

See merge request !14

.gitlab-ci.yml

 ---
-image: duneci/dune:2.6
-
 before_script:
   - duneci-install-module https://git.imp.fu-berlin.de/agnumpde/dune-matrix-vector.git
   - duneci-install-module https://git.imp.fu-berlin.de/agnumpde/dune-solvers.git
   - duneci-install-module https://git.imp.fu-berlin.de/agnumpde/dune-fufem.git
 
 dune-fufem--gcc:
+  image: duneci/dune:2.6-debian-10-gcc-7-14
   script: duneci-standard-test
 
 dune-fufem--clang:
-  script: duneci-standard-test --opts=/duneci/opts.clang
+  image: duneci/dune:2.6-debian-10-clang-7-libcpp-17
+  script: duneci-standard-test