Merge branch 'add-gitlab-ci' into 'master'

Add a minimal CI config file

See merge request hpc/hpc-bin!1

.gitlab-ci.yml

+build:
+  image: "registry.gitlab.haskell.org/ghc/ci-images/x86_64-linux-fedora33"
+  tags:
+    - "x86_64-linux"
+  script:
+    - "cabal build -w $GHC"