Add a step to verify if Docker is working as expected

Like in the other installation guides I added a simple test
to verify that importing images and starting containers is
working.

Docker-DCO-1.1-Signed-off-by: Christian Berendt <berendt@b1-systems.de> (github: berendt)

docs/sources/installation/openSUSE.md

@@ -47,6 +47,12 @@ Docker daemon.
 
     $ sudo usermod -G docker <username>
 
+To verify that everything has worked as expected:
+
+    $ sudo docker run -i -t b1systems/opensuse /bin/bash
+
+This should download and import the `opensuse` image, and then start `bash` in a container. To exit the container type `exit`.
+
 **Done!**
 
 Continue with the [User Guide](/userguide/).