In bash (outside docker):
docker run -d -p 8080:80 nextcloud

Inside container:
    1  cd config/
    2  ls
    3  nano config.php 
    4  apt install nano
    5  apt update
    6  apt install nano
    7  nano config.php 
    8  history

Add to config.php:

<?php
$CONFIG = array (
  'trusted_proxies'   => ['127.0.0.1'],
  'overwritehost'     => 'yourdomain.tld',
  'overwriteprotocol' => 'https',
  ...
);