The linked matter is not a bug. It was configuration matter. 
I initially chose Snappymail because of its positive reviews over that of the NextCloud mail app, however no matter how advantageous Snappymail may appear, it’s completely useless in my current operational scenario which I’m certain others who are adopting to Docker container usage will soon discover.
Until you provide more info (such as your Compose file) we’re just guessing here as to why you’re experiencing this problem. At the moment we don’t even know which Docker image you’re using (there are many).
There is nothing inherently special about a Docker environment re: snappymail in terms of storing settings – at least if the containers and snappymail are both set up fully.
It would also be helpful if you filled out the support template.