Enhance cleanup section

CONTRIBUTING.md

@@ -71,8 +71,21 @@ driven by the `RUNNER_ENTRYPOINT` variable in [`runner.sh`](./runner.sh).
 ## Cleanup
 
 During development, many images might be created. To clean them away, you can
-run the following command when using the `krunvm` runtime, or `podman image rm`:
+run one of the following commands.
+
+When using the `krunvm` runtime:
 
 ```bash
 buildah rmi $(buildah images --format '{{.ID}}')
 ```
+
+When using `podman+krun`:
+
+```bash
+podman image rm $(podman images -q)
+```
+
+> [!WARNING]
+> These commands will remove all unused images on your system. Make sure you
+> don't need any of these images for other projects before running the cleanup.
+> You may need to rebuild images for this project after cleanup.