fill-in build_system section

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

docs/developer_guide/tutorials/build_system/compile_sdk.md

@@ -1,3 +1,65 @@
+Starting from version 2.0.0, the pre-compiled FunKey SDK is available
+on [Github][1], and it is also compiled automatically as a first step
+when building the [full FunKey-OS distribution][2].
+
+However, if you want to compile the SDK only, here are the
+instructions below:
+
+## On a Physical Machine / In a Virtual Machine
+
+You may now build the SDK with:
+
+```bash
+$ make sdk
+```
+
+This may take a while (~1h), so consider getting yourself
+a cup, a glass or a bottle of your favorite beverage ;-)
+
+<ins>Note</ins>: you will need to have access to the network, since
+buildroot will download the package sources.
+
+After building, you should obtain the SDK file
+`FunKey-sdk-X.Y.Z.tar.gz` in the `images` directory.
+
+## In a Docker Container
+
+You may now build your FunKey with:
+```bash
+$ docker run --name funkey-os funkeyproject/funkey-os make sdk -C /home/funkey/FunKey-OS
+```
+
+Or alternatively, you can run it in the background with:
+```bash
+$ docker run -d --name funkey-os funkeyproject/funkey-os make sdk -C /home/funkey/FunKey-OS
+```
+
+If you launch it in the background, you can still follow what is going on with either:
+```bash
+$ docker top funkey-os
+```
+Or:
+
+```bash
+$ docker logs funkey-os
+```
+
+This may take a while (~1h), so consider getting yourself a cup, a
+glass or a bottle of your favorite beverage ;-)
+
+<ins>Note</ins>: you will need to have access to the network, since
+buildroot will download the package sources.
+
+After building, you can copy the SDK file `FunKey-sdk-X.Y.tar.gz` from
+the container into the host current directory:
+
+```bash
+$ mkdir images
+$ docker cp funkey-os:/home/funkey/FunKey-OS/images/FunKey-sdk-X.Y.Z.tar.gz images/
+```
+
+[1]: https://github.com/FunKey-Project/FunKey-OS/releases/
+[2]: ../compile_distribution
 
 --8<--
 includes/glossary.md