Howard Miller

As that is an optional plugin, you might do better to ask in their Github issues tracker. It's quite possible that nobody in these forums is familiar with this plugin (I'm certainly not).

As you obviously must know Docker well, I would recommend building your own stack for Docker (web server, php-fpm, database, redis and so on) and then installing the normal Moodle download.

The link you found is for the Moodle Mobile App, not Moodle.

You only see Download because the web server (or, to be more precise, the web server / php-fpm user) does not have permission to write to the Moodle program files. You need to change the file ownership and/or permissions.

As an aside, I would *never* leave the program files writeable on a production site. So, if this is a production site, I would say that your situation is a good thing. I would only upgrade plugins this way on a test site and then transfer to the live site using (for example) Git.