build-sys: Keep bootc-pkg image for layer caching

Removing localhost/bootc-pkg at the end of the package target
also deletes the build stage layers, causing subsequent builds
to miss the cache and rebuild the RPMs from scratch.

Keep the image around; use `just clean-local-images` to reclaim space.

Signed-off-by: Colin Walters <walters@verbum.org>

Author: cgwalters

Justfile

@@ -121,7 +121,7 @@ package: _packagecontainer
     podman run --rm localhost/bootc-pkg tar -C /out/ -cf - . | tar -C target/packages/ -xvf -
     chmod a+rx target target/packages
     chmod a+r target/packages/*.rpm
-    podman rmi localhost/bootc-pkg
+    # Keep localhost/bootc-pkg for layer caching; use `just clean-local-images` to reclaim space
 
 # Build+test using the `composefs-sealeduki-sdboot` variant.
 test-composefs: