[!291] Switch to clang 7

Merge branch 'feature/fix-ci' into 'master'

ref:extensions/dune-codegen See whether that helps

See merge request [extensions/dune-codegen!291]

  [extensions/dune-codegen!291]: gitlab.dune-project.org/extensions/dune-codegen/merge_requests/291

.gitlab-ci.yml

@@ -19,7 +19,7 @@ git-debian-10-gcc-7-14:
     - "perftool"
 
 git-debian-10-clang-6-libcpp-17:
-  image: "registry.dune-project.org/docker/ci/dune-pdelab:git-debian-10-clang-6-libcpp-17"
+  image: "registry.dune-project.org/docker/ci/dune-pdelab:git-debian-10-clang-7-libcpp-17"
   script: duneci-standard-test
   tags:
     - "cores:20"