Hello, @khsu
I believe that he question that you have here is similar to an existing topic that was raised before, which was related to Docker deployment with HTTPS.
May I know if you have can have a look at NGINX as a proxy option for your Mattermost Docker installation by referring to the steps in How do I setup an NGINX proxy with the Mattermost Docker installation?
Additionally, you may want to take a look at this article Mattermost Server Docker Installation Tutorial as well as it provides a guide to do this too.