Don't build documentation on STABLE nightlies (partially addresses comments at...

Don't build documentation on STABLE nightlies (partially addresses comments at bob/bob.io.image./bin/fab -f staging web)

gitlab/build.sh

@@ -19,7 +19,7 @@ if [ -x ./bin/bob_dbmanage.py ]; then
   run_cmd ./bin/bob_dbmanage.py all download --missing;
 fi
 
-if [ -d ./doc ]; then
+if [ -d ./doc ] && [ "${STABLE}" != "true" ]; then
   run_cmd ./bin/sphinx-build -W doc sphinx
 fi