Update .gitlab.ci to deploy the project only on main branch

.gitlab-ci.yml

@@ -36,6 +36,8 @@ test:
     - docker-compose -f docker-compose.ci.yml run web sh -c "poetry run python manage.py makemigrations && poetry run python manage.py migrate && poetry run python manage.py test core -v 2"
 
 deploy:
+  only:
+    - main
   stage: deploy
   needs:
     - test