L2SS-218 Disable exit on error in clean_images

This allows make to return with a non-0 code which in turn does
not force Bash to exit.  This happens when the containers where
already down and/or the volumes cleaned up.

bootstrap/sbin/rebuild_system_from_scratch.sh

@@ -48,8 +48,14 @@ function stop_images()
 
 function clean_images()
 {
+    # This can return a non-0 return code if the system
+    # has already been shut down.
+    # Therefore disable Bash's exit on error flag
+    set +e
     pushd ${HOME_DIR}/docker-compose && make clean
     popd
+    # And enable it again.
+    set -e
 }
 
 function start_minimal_tango()