For true zero downtime, the connections have to be slowly drained, i.e. two all instances may exist at the same time. Does coolify support that?

Coolify does support zero downtime deployments, but the documentation isn't live yet: https://github.com/coollabsio/documentation-coolify/blob/640...

It uses docker stop once the new container is healthy with a 30 second timeout, which I believe lets existing connections drain out.