update.sh: extract local tarball (#66)
update.sh: extract local tarball (#66)

Rules `ADD` follows:
- If `<src>` is a URL and `<dest>` does not end with a trailing slash, then a file is downloaded from the URL and copied to `<dest>`.

- If `<src>` is a URL and `<dest>` does end with a trailing slash, then the filename is inferred from the URL and the file is downloaded to `<dest>/<filename>`. For instance, `ADD http://example.com/foobar /` would create the file `/foobar`. The URL must have a nontrivial path so that an appropriate filename can be discovered in this case (http://example.com will not work).
multiarch/qemu-user-static