ci: automatic push to github

- issue solved with https://stackoverflow.com/questions/63451263/sync-two-git-repositories

Refs #11

.gitlab-ci.yml

@@ -67,4 +67,4 @@ github:
     - git config --global user.name "Gitlab CI"
   script:
     - git remote add github https://github.com/inrae/hubeau.git
-    - git push github ${CI_COMMIT_BRANCH}
+    - git push --mirror github