Fix build stage

.gitlab-ci.yml

@@ -2,6 +2,18 @@ default:
  tags: ['docker']   
  image: registry.gitlab.inria.fr/solverstack/docker/distrib
 
+stages:
+  - build
+  - pages
+
+build:
+  stage: build
+  artifacts:
+    name: cpp_tools_build
+    expire_in: 1 day
+    paths:
+      - build/
+
 doc:
   stage: build 
   artifacts: