Be picky about sphinx doc generation

gitlab/build.sh

@@ -10,7 +10,7 @@ if [ -x ./bin/bob_dbmanage.py ]; then
 fi
 
 if [ -d ./doc ]; then
-  run_cmd ./bin/sphinx-build doc sphinx
+  run_cmd ./bin/sphinx-build -n -W doc sphinx
 fi
 
 if [ -z "${WHEEL_TAG}" ]; then