Comment 7 for bug 1799895

Se my previous attempt for adding **individual** caches to containers update https://review.openstack.org/#/c/575742/4/scripts/container-update.py@146

but I'm not sure it'd be good idea to use a common cache with concurrent workers doing updates there. Doing updates of containers serialized would allow having a shared for all cache... But updating 50+ containers one by one, even with shared cache, might take even more time in the end.