Well the Snille/MMM-homeassistant-sensors module just says:

Installation
In your terminal, go to your MagicMirror's Module folder:

cd ~/MagicMirror/modules
Clone this repository:

git clone https://github.com/Snille/MMM-homeassistant-sensors
Enter the folder:

cd MMM-homeassistant-sensors
Install Node-Modules (The MaterialDesignIcons webfont icon names can be used.).

npm install

and the leinich/MMM-homeassistant-sensors module just says:

Navigate into your MagicMirror's modules folder and clone this repository: cd ~/MagicMirror/modules && git clone https://github.com/leinich/MMM-homeassistant-sensors.git

If you want to use icons for the sensors download the MaterialDesignIcons webfont from https://github.com/Templarian/MaterialDesign-Webfont/archive/master.zip and unzip the folder: cd ~/MagicMirror/modules/MMM-homeassistant-sensors && wget https://github.com/Templarian/MaterialDesign-Webfont/archive/master.zip && unzip master.zip

I tried downloading and unzipping the master.zip file and running the npm install command in the directory (in the docker container) as you suggested. After restarting my MagicMirror2 container, nothing was any different.

Clearly there is some ‘piece of the jigsaw’ that i’m missing.