gitlab-ci: Bump docker image

Bizarrely, the nixos/nix:2.3 tag still pointed to an image that is
over two years old, while nixos/nix:2.3.13 is much newer. Use
the latter.

.gitlab-ci.yml

@@ -22,7 +22,7 @@ stages:
 
 variables:
   # Which nixos/nix Docker image tag to use
-  DOCKER_TAG: "2.3"
+  DOCKER_TAG: "2.3.12"
 
   # Default GHC bindist
   GHC_TARBALL: "https://gitlab.haskell.org/api/v4/projects/1/jobs/artifacts/master/raw/ghc-x86_64-fedora27-linux.tar.xz?job=validate-x86_64-linux-fedora27"