`bootc container lint` checks `rm -rf /var/cache /var/lib/dnf` in Containerfile

To avoid error `error: ostree-tar: Processing deferred hardlink var/cache/dnf/rhel-appstream-c101d4db5fbc3a4f/repodata/527fa9e5c8c45b22a7bbc2821c96540817984e837c219acb5141a462a08d45f6-primary.xml.gz: Failed to find object: No such file or directory: 527fa9e5c8c45b22a7bbc2821c96540817984e837c219acb5141a462a08d45f6-primary.xml.gz: Processing tar: Failed to commit tar: ExitStatus(unix_wait_status(256))`, `rm -rf /var/cache /var/lib/dnf` in Containerfile should be checked and report warnings.

Start with the `bootc container lint` command and reproduce the reported ostree-tar failure using a Containerfile containing `rm -rf /var/cache /var/lib/dnf`. Trace how lint checks Containerfiles, then add the warning behavior and verify that this pattern is reported without obscuring the underlying error.

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