GitLab CI: some investigation, use custom Docker img

.gitlab-ci.yml

-image: ubuntu:18.04
+image: league/haskell-stack:20180605
 
 # Here are two problems we're trying to solve with this before_script:
 # https://github.com/commercialhaskell/stackage/issues/3132
@@ -12,17 +12,19 @@ image: ubuntu:18.04
 # need build-essential.
 
 before_script:
-  - apt-get -y update
-  - apt-get -y install netbase curl
-  - curl -sSL https://get.haskellstack.org/ | sh
+  - echo USER IS $(whoami)
+  - echo HOME IS $HOME
+  - ls -l /
 
 build:
   # For some reason, just `stack` uses the old version, so explicitly
   # ask for the new one from ~/.local:
   script:
-    - stack build
+    - echo Fake build
+    - echo PWD is $(pwd)
+    - mkdir .stack-work
+    - touch .stack-work/SAVED
 
 cache:
   paths:
-    - /root/.stack
-    - /home/bookme/.stack-work
+    - .stack-work