Use the "domain name setup" image (previously unused) in the gitian docs

Author: practicalswift

doc/gitian-building.md

@@ -131,6 +131,7 @@ To select a different button, press `Tab`.
   - Leave domain name empty.
 
 ![](gitian-building/debian_install_5_configure_the_network.png)
+![](gitian-building/debian_install_6_domain_name.png)
 
 - Choose a root password and enter it twice (remember it for later)