Fix GHCR build workflow to fail on errors

- Remove interactive shell exec from Dockerfile test script
- Add explicit error handling with set -e in workflow
- Add explicit exit code checks for docker run commands
- Improve error messages when tests fail

Co-authored-by: johndoe6345789 <224850594+johndoe6345789@users.noreply.github.com>

Dockerfile

@@ -90,7 +90,6 @@ echo ""
 echo "To test the init system:"
 echo "  docker run --rm <image> /sparkos/rootfs/sbin/init --help"
 echo ""
-exec /bin/sh
 EOF
 
 RUN chmod +x /sparkos/test.sh