Mark `docker/tarfile`'s `GetBlob` thread-safe
Mark `docker/tarfile`'s `GetBlob` thread-safe

This fixes #567, and *should* be safe from what I can tell (according to the
reasoning I mention in that issue).

Signed-off-by: Teo Klestrup Röijezon <437a1c14efaa8e9881ef6bb077411dc1d24cb4c0@nullable.se>

containers/image,containers/image