Add general system update and Imagemagick for Debian

docs/en/installation/debian.md

@@ -8,11 +8,20 @@ Note that 'sudo' is not installed by default in Debian. It's possible to install
 su
 ```
 
+
 > For [Vagrant](/en/installation/vagrant.md) run:
 > ```
 > sudo su -
 > ```
 
+## System update
+
+Run a general system update:
+
+```bash
+apt-get update
+```
+
 ## Git
 
 Git is officially maintained in Debian:
@@ -129,6 +138,14 @@ createuser consul --createdb --superuser --pwprompt
 exit
 ```
 
+## Imagemagick
+
+Install Imagemagick:
+
+```bash
+apt-get install imagemagick
+```
+
 ## ChromeDriver
 
 To run E2E integration tests, we use Selenium along with Headless Chrome.