add example for how to install modules

[phohomi]

Hi,

Drupals power comes from it's large library of modules, but there is no description of how to install a module. Only how to mount a folder with modules, and it is no longer advised to download and unzip them manually.

A dockerfile based on this official image in which composer require ... is demonstrated, together with a docker file that builds this dockerfile: wouldn't that be something to add to the documentation?

Currently when you search for Drupal and docker, you find a lot of alternative docker images, and very few examples of how to work with the official image in a best practice fashion.