[CI] Also test against Dune 2.7

Even for the master branch I'd like to know that this kinda works against the 2.7 release, because that is what people using packages have.

.gitlab-ci.yml

@@ -6,6 +6,11 @@ before_script:
   - duneci-install-module https://git.imp.fu-berlin.de/agnumpde/dune-fufem.git
   - duneci-install-module https://git.imp.fu-berlin.de/agnumpde/dune-solvers.git
 
+dune:2.7 gcc:
+  image: registry.dune-project.org/docker/ci/dune:2.7-debian-10-gcc-8-17
+  before_script:
+  script: duneci-standard-test
+
 dune:git  clang  C++17:
   image: registry.dune-project.org/docker/ci/dune:git-debian-10-clang-7-libcpp-17
   script: duneci-standard-test