Pin the Docker image version for .build as well

.gitlab-ci.yml

@@ -19,7 +19,7 @@ stages:
   - deploy
 
 variables:
-  # Commit of ghc/ci-images repository from which to pull Docker images
+  # Which nixos/nix Docker image tag to use
   DOCKER_TAG: "2.3"
 
   # Default GHC bindist
@@ -78,7 +78,7 @@ build-8.10:
   tags:
     - x86_64-linux
 
-  image: nixos/nix
+  image: "nixos/nix:$DOCKER_TAG"
 
   cache:
     key: build-HEAD