Rename "container" to "image"

- Updated `wwctl upgrade` to handle updates
- Maintained `.Container` and `.ContainerName` in tstruct
- Added `ContainerName()` methods to node and profile objects
- Added `--container`, `-C` aliases to wwctl commands (`<node|profile> <add|set>`)
- Added `wwctl container` alias
- Added support for `container_exit.sh` if `image_exit.sh` is not found

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

README.md

@@ -55,9 +55,7 @@ many thousands of compute nodes. Generally speaking, operating system
 state introduce a surface for potential discrepancies and version
 creep between nodes and thus Warewulf has always gone with the "single
 system image" approach to clustered operating system management.  This
-means that you can have a single management node "image". For the last
-20 years, we've been calling this a Virtual Node File System (VNFS),
-but in hindsight, this is a container.
+means that you can have a single node "image".
 
 At its core, Warewulf v4 focuses on what has made Warewulf so widely
 loved: simplicity, ultra scalable, lightweight, and an easy to manage