Adds Docker for firmware-building

This commit adds a Dockerfile to build a docker container to build the firmware with. It also updates the documentation on how to do that.

Documentation/how-to-build.rst

@@ -111,3 +111,25 @@ In order to do a rebuild you can issue a clean command to ninja via
   $ ninja -C build/ -t clean
 
 Otherwise, rerunning ``./bootstrap.sh`` will also clean the build-directory.
+
+Docker
+======
+Alternatively, clone the ``master`` branch of the firmware repository and enter it:
+
+.. code-block:: shell-session
+
+   $ git clone https://git.card10.badge.events.ccc.de/card10/firmware.git
+   $ cd firmware
+
+Afterwards, build a docker-container which will build the firmware via:
+
+.. code-block:: shell-session
+
+   $ docker build -f Dockerfile_fwbuild -t card10-firmware-builder .
+
+Now, you can start the container with the firmware directory mounted, which will build the 
+firmware into the firmware/build directory:
+
+.. code-block:: shell-session
+
+   $ docker run -v $(pwd):/firmware card10-firmware-builder