Also document composer setup

Signed-off-by: Marcel Robitaille <mail@marcelrobitaille.me>

docs/dev/misc/automated-testing/index.md

@@ -22,9 +22,12 @@ The [overview of the backend testing framework](backend/) should give a general
 These instructions will help you quickly get set up for testing using the default options.
 If you would like to learn more or use non-default options, you may read [Overview over the backend testing implementation](./backend/index.md).
 
-1. Open testing directory: `cd .github/actions/run-tests/`.
 1. Install [Docker](https://www.docker.com/) (e.g. `sudo apt install docker`). Nextcloud will be installed in Docker for testing.
+1. Install [PHP](https://www.php.net/) and some extensions: `sudo apt install php php-zip php-xml`
+1. Install [Composer](https://getcomposer.org/), the PHP package manager (e.g. `sudo apt install composer`)
 1. Although this app is written in PHP and JavaScript, the test helpers are written in Python. Please [install Python 3](https://www.python.org/downloads/).
+1. Install PHP dependencies: `composer install`
+1. Open testing directory: `cd .github/actions/run-tests/`.
 1. Create a Python virtual environment: `python -m venv venv`.
 1. Activate the virtual environment: `source venv/bin/activate`.
 1. Install Python dependencies: `pip install -r ./requirements.txt`.