Update the image addresses in .gitlab-ci.yml

From

  duneci/dune:***       (from Docker Hub)

to

  registry.dune-project.org/docker/ci/dune:***

(from the Dune homepage).  The mirror at Docker Hub is not always
up-to-date.

.gitlab-ci.yml

@@ -29,33 +29,33 @@ before_script: &before
     EOT
 
 dune:2.6 gcc:
-  image: duneci/dune:2.6-debian-10-gcc-8-17
+  image: registry.dune-project.org/docker/ci/dune:2.6-debian-10-gcc-8-17
   script: duneci-standard-test
   variables:
     DUNECI_BRANCH: releases/2.6-1
 
 dune:2.6 clang:
-  image: duneci/dune:2.6-debian-10-clang-7-libcpp-17
+  image: registry.dune-project.org/docker/ci/dune:2.6-debian-10-clang-7-libcpp-17
   script: duneci-standard-test
   variables:
     DUNECI_BRANCH: releases/2.6-1
 
 dune:git gcc:
-  image: duneci/dune:git-debian-10-gcc-8-17
+  image: registry.dune-project.org/docker/ci/dune:git-debian-10-gcc-8-17
   before_script:
   - *patch-dune-common
   - *before
   script: duneci-standard-test
 
 dune:git clang:
-  image: duneci/dune:git-debian-10-clang-7-libcpp-17
+  image: registry.dune-project.org/docker/ci/dune:git-debian-10-clang-7-libcpp-17
   before_script:
   - *patch-dune-common
   - *before
   script: duneci-standard-test
 
 dune:git parmg gcc:
-  image: duneci/dune:git-debian-10-gcc-8-17
+  image: registry.dune-project.org/docker/ci/dune:git-debian-10-gcc-8-17
   before_script:
   - *patch-dune-common
   - *before
@@ -63,7 +63,7 @@ dune:git parmg gcc:
   script: duneci-standard-test
 
 dune:git parmg clang:
-  image: duneci/dune:git-debian-10-clang-7-libcpp-17
+  image: registry.dune-project.org/docker/ci/dune:git-debian-10-clang-7-libcpp-17
   before_script:
   - *patch-dune-common
   - *before