Set up CI pipelines

.gitlab-ci.yml

+---
+# Install external dependencies
+before_script:
+  - duneci-install-module https://gitlab.dune-project.org/fufem/dune-matrix-vector.git
+  - duneci-install-module https://git.imp.fu-berlin.de/agnumpde/dune-solvers.git
+  - duneci-install-module https://gitlab.dune-project.org/fufem/dune-fufem.git
+
+dune:2.9 debian-11 gcc-10 C++20:
+  variables:
+    DUNECI_BRANCH: releases/2.9
+  image: registry.dune-project.org/docker/ci/dune:2.9-debian-11-gcc-10-20
+  before_script:
+  script: duneci-standard-test
+
+dune:git ubuntu-20-04 clang-10  C++20:
+  image: registry.dune-project.org/docker/ci/dune:git-ubuntu-20.04-clang-10-20
+  script: duneci-standard-test
+
+dune:git debian-11 gcc-10  C++20:
+  image: registry.dune-project.org/docker/ci/dune:git-debian-11-gcc-10-20
+  script: duneci-standard-test