.gitlab-ci.yml: switch to new CI images

Closes: #12
(cherry picked from commit 66af8250)

.gitlab-ci.yml

 ---
-dune:git  clang:
-  image: duneci/dune:git
-  script: duneci-standard-test --opts=/duneci/opts.clang
+dune:2.6 debian:10 clang-7-libcpp-17:
+  image: registry.dune-project.org/docker/ci/dune:2.6-debian-10-clang-6-libcpp-17
+  script: duneci-standard-test
   tags: [duneci]
 
-dune:git  gcc:
-  image: duneci/dune:git
+dune:2.6 debian:10 gcc-8:
+  image: registry.dune-project.org/docker/ci/dune:2.6-debian-10-gcc-8-17
   script: duneci-standard-test
   tags: [duneci]
 
-dune:2.6  gcc:
-  image: duneci/dune:2.6
+dune:2.6 debian:9 gcc-6:
+  image: registry.dune-project.org/docker/ci/dune:2.6-debian-9-gcc-6-14
   script: duneci-standard-test
   tags: [duneci]