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/contents/containers.rst

@@ -48,7 +48,7 @@ Here is an example of importing from Docker Hub.
 
 .. code-block:: console
 
-   # wwctl container import docker://warewulf/rocky rocky-8
+   # wwctl container import docker://ghcr.io/hpcng/warewulf-rockylinux:8 rocky-8
    Getting image source signatures
    Copying blob d7f16ed6f451 done
    Copying config da2ca70704 done

userdocs/contributing/development-environment-kvm.rst

@@ -102,7 +102,7 @@ Build and install Warewulf on wwdev
    sudo wwctl configure ssh  # Build the basic ssh keys to be included by the default system overlay
 
    # Pull and build the VNFS container and kernel
-   sudo wwctl container import docker://warewulf/centos-8 centos-8 --setdefault
+   sudo wwctl container import docker://ghcr.io/hpcng/warewulf-centos:7 centos-7 --setdefault
    sudo wwctl kernel import build $(uname -r) --setdefault
 
    # Set up the default node profile

userdocs/contributing/development-environment-vbox.rst

@@ -117,7 +117,7 @@ I have VirtualBox running on my desktop.
    sudo wwctl configure ssh  --persist # Build the basic ssh keys to be included by the default system overlay
 
    # Pull and build the VNFS container and kernel
-   sudo wwctl container import docker://warewulf/centos-7 centos-7 --setdefault
+   sudo wwctl container import docker://ghcr.io/hpcng/warewulf-centos:7 centos-7 --setdefault
    sudo wwctl kernel import build $(uname -r) --setdefault
 
    # Set up the default node profile

userdocs/quickstart/el7.rst

@@ -103,7 +103,7 @@ default running kernel from the controller node and set both in the
 
 .. code-block:: bash
 
-   sudo wwctl container import docker://warewulf/centos-7 centos-7 --setdefault
+   sudo wwctl container import docker://ghcr.io/hpcng/warewulf-centos:7 centos-7 --setdefault
    sudo wwctl kernel import $(uname -r) --setdefault
 
 Set up the default node profile

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