test with alpine image rather than registry image

.gitlab-ci.yml

-#image: python:3.7-alpine
-image: registry.forgemia.inra.fr/umr-gdec/magatt
+image: python:3.7-alpine
+#image: registry.forgemia.inra.fr/umr-gdec/magatt
 
 # global before script, set venv in all job
 before_script:
-    - source /opt/venv/cicd/bin/activate
+    - pip install -U sphinx
+    - pip install -U sphinx-rtd-theme
+    - pip install -U myst-parser
+
 
 test:
   stage: test