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>

overlays/wwinit/internal/wwinit_test.go

@@ -85,7 +85,7 @@ nfs:
 const wwinit_config string = `backupFile: true
 writeFile: true
 Filename: warewulf/config
-WWCONTAINER=rockylinux-9
+WWIMAGE=rockylinux-9
 WWHOSTNAME=node1
 WWROOT=initramfs
 WWINIT=/sbin/init