Docker is nice for things that have complex installations and I want a very specific implementation that I don’t plan to tweak very much. Otherwise, it’s more hassle than it’s worth. There are lots of networking issues like limited/experimental support for IPv6, and too much is hidden and preconfigured, making it difficult to make adjustments that would otherwise just be a config file change.

So it is good for products like a mail server where you want to use the exact software they use like let’s say postfix + dovecot + roundcube + nginix + acme + MySQL + spam assassin + amavisd, etc. But you want to use an existing reverse proxy and cert it setup, or want to use a different spam filter or database and it becomes a huge hassle.