As pointed out in #1459, docker cli fails to detect that the input is a tarball,
in case it is generated by `git archive --format=tgz`.
This happens because `git archive` adds some metadata to the initial tar header,
and so it is more than 1 block (of 512 bytes) long, while we only provide 1 block
to archive/tar.Next() and it fails.
To fix, give it 2 blocks :)
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>
0c20554f69