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>

internal/app/api/API.md

@@ -14,12 +14,12 @@ wwctl node delete
 wwctl node list
 wwctl node set
 wwctl node status
-wwctl container build
-wwctl container delete
-wwctl container import
-wwctl container list
-wwctl container show
-wwctl container copy
+wwctl image build
+wwctl image delete
+wwctl image import
+wwctl image list
+wwctl image show
+wwctl image copy
 
 Some notes on the files: