Add building docker images to the installation instructions

docs/source/installation.rst

@@ -15,6 +15,10 @@ You start with checking out the source code, f.e. the master branch, as well as
   git submodule init
   git submodule update
 
+Next, we build our software as docker images. This may take a while::
+
+  make build
+
 To start the software, just start the containers::
 
   make start