Have .dockerignore support Dockerfile/.dockerignore

If .dockerignore mentions either then the client will send them to the
daemon but the daemon will erase them after the Dockerfile has been parsed
to simulate them never being sent in the first place.

Closes #8330

Signed-off-by: Doug Davis <dug@us.ibm.com>
I take that back... this change is ok. :-)
This only skips the metadata of the top-level dir being copied which is fine because we don't actually try to ADD/COPY that dir, just its contents/children.  So, excluding it from the hash calc should not cause an issue.  I'll add a comment to the code.