Hi,
I'm using Docker too. What I did was expose the port, so that it looks like the host machine "is" the mattermost server.
Something along the lines of
docker run -p 80:8065 mattermostimage
I believe the exposed port from docker image is 8065, but you can adjust as needed.
If you run it like that, then go to your host IP (or name) and you should be able to get to mattermost.