I have commented the
you cited - look there for details.
You can't have two server apps on the same port. Mattermost is already using 8065. You'll need to change this or use another port externally.
You need to have a virtual host config file for the mattermost server access, use there if you want mattermost be accessible there.
Change the mattermost port in config.json to something different from 8065 and the address from 0.0.0.0 to 127.0.0.1 to prevent non ssl-access.