If you have a two container install, the new container builds while the old one runs. Downtime is just the amount of time it takes to launch the new container. The only issue is that you need enough ram to build a container while the other runs.
Move from standalone container to separate web and data containers, but I usually move a new vm.
If you want zero down time then you need a load balancer that keeps the old container running until the new one has fully started. Then you shut down the old container and do the post update migrations.