Hi @farhaadn ,
If you want to use an external nginx, you can do so by using the following yml files to start the Mattermost docker environment:
docker-compose -f docker-compose.yml -f docker-compose.without-nginx.yml up -d
The listening port should be left untouched, the application will listen on port 8065 then and it will be unencrypted.
The nginx proxy manager then needs to connect to the Mattermost backend, which should then be available on http://localhost:8065 and by accessing the service through your nginx proxy manager, you will then also have the encryption and public IP active.