What you can learn from this is that when using appliances like Mailcow, it is best to follow the recommendations in the documentation of the respective products.
That being said, I don’t think it’s prohibited to have a discussion here about how well Mailcow runs on non-LTS distributions or with different Docker versions, but I don’t think you’ll find many discussion partners for that ;-) That’s simply because there is no advantage of trying this, and therefore there are only few people who have experience with it.
Most people don’t play around with their mail servers more than necessary, they just want them to work as reliably as possible. That’s why they usually don’t use a rolling release distribution, or non-LTS Ubuntu releases that you have to upgrade every six months, which creates expenses and, in the worst case, downtime if something goes wrong.
So just grab the latest Ubuntu LTS or the latest Debian, install the latest Docker engine and Docker Compose from the Docker repos as recommended here, and everything will just work.