split pipeline in 2 step build & deploy

.gitlab-ci.yml

+image: registry-gitlab.pasteur.fr/hub-courses/sphinx-env/sphinx:1.4.1
+
+sphinx:
+  stage: build
+  script:
+    - make html
+    - mv build/html/ .
+  artifacts:
+    paths:
+      - html/
 
 pages:
   stage: deploy
-  image: registry-gitlab.pasteur.fr/hub/courses/sphinx-env/sphinx:1.4.1
+  dependencies:
+    - sphinx
   script:
-    - make html
-    - mv build/html/ public/
+    - mv html/ public/
   artifacts:
     paths:
       - public
   only:
-    - master
+    - master
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