additional hint when building a Singularity container

doc/container/singularity.rst

@@ -20,7 +20,8 @@ Singularity
 We do not provide a "standalone" Singularity image, but rather bootstrap from a 
 Docker image.
 
-Build the image with:
+Change to the directory containing the Singularity file
+(<PROMOD3_CHECKOUT>/container) and build the image with:
 
 .. code-block:: bash