tell it what docker image to use

.gitlab-ci.yml

@@ -32,6 +32,9 @@ morph-builds:
     # This is far from ideal but maybe it's a tolerable hack for now.
     - "nixos"
     - "docker"
+
+  image: "nixos/nix:latest"
+
   stage: "test"
   variables:
     # CACHIX_AUTH_TOKEN, which lets us push to cachix, is supplied by GitLab