Hey,
So the doc you're referring to assumes OS reinstallation for hosts during the upgrade. As you've rightfully mentioned - upgrade of host OS does not actually result in LXC containers upgrade.
There are multiple ways forward with that. Eventually, you can run do-release-upgrade inside of containers independently from the host.
Or, you can re-create the containers per-control. In order to re-create all containers on a specific control node, you can run:
* openstack-ansible lxc-containers-destroy.yml --limit ${control}-host_containers
* openstack-ansible lxc-containers-create.yml --limit ${control}-host_containers,${control}
So merging that with the doc mentioned, Deploying Infrastructure Hosts section specifically, you need to replace step 2 with the commands above.