The docs simultaneously say:
Cloudron package does not support installing plugins via Composer since that involves editing composer.json which makes automatic updates tricky.
Some plugin release tarballs do not contain dependancies and have to be installed via composer.
If installing plugins via compser isn't supported, why are instructions on how to do it given? (and why don't they involved editing composer.json given that's supposedly why it's not supported)
Just rewording the docs a bit. But what it's trying to say is:
Per https://plugins.roundcube.net/#/ , it says to install plugins by editing
composer.json. This won't work because composer.json is readonly in Cloudron package.
Plugins have dependencies listed in their own composer.json . So the installation instructions are to install the deps of plugins (when installing plugin from source).