[vcvkk]

Motivation

• Prevent absolute symlinks in a rootfs from baking host paths into an exported archive and from being rejected by Python's safe extraction filter.
• Keep exported archives portable and restorable such that rootfs-local links still resolve correctly after extraction.

Description

• Added a _portable_link filter that converts rootfs-local absolute symlink targets into relative targets using posixpath.relpath when creating tar members.
• Use the filter by passing filter=_portable_link to tarfile.add so symlinks are stored as relative links while ordinary files and links are unchanged.
• Imported posixpath and updated the archive docstring and a unit test expectation to reflect the new behavior.

Testing

• Ran the rootfs export tests in tests/test_rootfs.py including test_the_archive_holds_the_container, test_a_symlink_is_stored_as_itself, test_the_archive_can_be_unpacked_back, and test_progress_is_reported_and_ends_at_one.
• All tested cases passed (the symlink test now expects the stored linkname to be "busybox").

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Codex Task