[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