[CI] add simple .gitlab-ci

Modify app to return exit code when error happened.

.gitlab-ci.yml

+# Optimising CI speed by using tips from https://blog.nimbleways.com/let-s-make-faster-gitlab-ci-cd-pipelines/
+image: cgenie/gargantext:9.6.6
+
+stages:
+  - test
+
+cabal:
+  stage: test
+  script:
+    - nix --experimental-features 'nix-command flakes' build
+    - nix --experimental-features 'nix-command flakes' run
+  allow_failure: false

app/Main.hs

@@ -2,12 +2,15 @@ module Main where
 
 import ISTEX
 import ISTEX.Client
+import System.Exit
 
 main :: IO ()
 main = do
   res <- getMetadataWith "artificial intelligence" (Just 10)
   case res of
-    (Left err) -> print $ "Error: " <> show err
+    (Left err) -> do
+      print $ "Error: " <> show err
+      exitWith $ ExitFailure 1
     (Right val) -> do
       print $ take 5 $ _documents_hits val
        -- print $ _abstract <$> (_hits val)