Update documentation to reference ghcr rather than DockerHub

Warewulf sample node images are now built automatically with GitHub CI/CD,
targeting ghcr.io. This updates the documentation to reference that location
rather than DockerHub.

Addresses #732

Signed-off-by: Jonathon Anderson <janderson@ciq.co>

userdocs/quickstart/el8.rst

@@ -105,7 +105,7 @@ default running kernel from the controller node and set both in the
 
 .. code-block:: bash
 
-   sudo wwctl container import docker://warewulf/rocky:8 rocky-8
+   sudo wwctl container import docker://ghcr.io/hpcng/warewulf-rockylinux:8 rocky-8
 
 
 Set up the default node profile