Create and chown `ROUNDCUBEMAIL_TEMP_DIR` although `config.inc.php` exists.
Reasoning:
1. A new container with `config.inc.php` will not have the default temp folder what will lead to errors even when `$config['temp_dir']` is left at its default value. If the `config.inc.php` specifies a custom one, the admin should now what he is doing and can set `ROUNDCUBEMAIL_TEMP_DIR` accordingly.
2. In a multi server setup a bind mount to `/tmp/roundcube-temp` will get the correct permissions set which will often be `root:root` with `0755` initially.
