Hey,
don’t use mattermost-docker at all. The compose file i provided will use the prebuilt images from Mattermost staff and should work out of the box. Right now this isn’t working because the mattermost-docker repo uses an non standard port.
You probably forgot to define the proxy_cache but without seeing your nginx.conf i can’t say where the problems are. My advice is to use the configuration i provided because this should work.