test ubuntu 20.04 toolchain

.gitlab-ci.yml

@@ -47,6 +47,12 @@ ubuntu:18.04 clang-6-17:
   variables:
     DUNECI_TOOLCHAIN: clang-6-17
 
+ubuntu:20.04 gcc-9-20:
+  <<: *common
+  image: registry.dune-project.org/docker/ci/ubuntu:20.04
+  variables:
+    DUNECI_TOOLCHAIN: gcc-9-20
+
 # specific job for testing sequential library version
 debian:10 gcc-8-17 sequential:
   <<: *common