fill-in build_system section

Signed-off-by: Michel-FK <michel.stempin@funkey-project.com>

docs/developer_guide/tutorials/build_system/get_sources.md

@@ -0,0 +1,38 @@
+## On a Physical Machine / In a Virtual Machine
+
+When using either physical Linux machine or virtual Linux machines
+(VirtualBox or WSL2), you must clone the FunKey OS repository from
+Github (here we place it into a `FunKey-OS` directory):
+
+```bash
+$ git clone https://github.com/FunKey-Project/FunKey-OS.git FunKey-OS
+```
+
+Then enter into the newly created directory:
+
+```bash
+$ cd FunKey-OS
+```
+## In a Docker Container
+
+When using a Docker container, you must first create a new directory
+(here we create a `FunKey-OS` directory) and get the FunKey-OS
+[Dockerfile][1] in it:
+
+```bash
+$ mkdir FunKey-OS
+$ cd FunKey-OS
+$ wget https://raw.githubusercontent.com/FunKey-Project/FunKey-OS/master/docker/Dockerfile -o Dockerfile
+```
+
+You must then build the docker image (don't forget the final dot!):
+
+```bash
+$ docker build -t funkeyproject/funkey-os .
+```
+
+[1]: https://github.com/FunKey-Project/FunKey-OS/blob/master/docker/Dockerfile
+
+--8<--
+includes/glossary.md
+--8<--