This project allows building a simple docker image of Mattermost. It does not include the databse.
The Mattermost configuration can be changed by setting MM_* environment variables, as explained in the Mattermost documentation:
Find the setting in config.json. In this case, ServiceSettings.SiteURL.
Add MM_ to the beginning and convert all characters to uppercase and replace the . with _. For example, MM_SERVICESETTINGS_SITEURL.
The setting becomes export MM_SERVICESETTINGS_SITEURL="http://example.com".
https://docs.mattermost.com/administration/config-settings.html