It looks like COPY does not untar files when they're coming from a
previous stage. It would be good if it would.

Use case: I'm trying to minimize an ubuntu image by creating a tgz with
some parts left out in one stage, then using the tgz in a scratch stage.
This is to avoid the problem of using COPY which loses ownership and
set[ug]id information. The thing is that I cannot use ADD since it
doesn't have a --from; I cannot use COPY since it won't unpack the
tgz; and I cannot copy and then unpack the tgz since there is no
/bin/sh so a RUN fails too (complaining that there is no /bin/sh).