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>
@duglin What *should* happen if I put in `includes`: `*ocker*` ? In that case `excludes` prevails but not if the line exactly matches .dockerignore or Dockerfile ?

I tend to think this is not desirable because it would be inconsistent, however I don't have a good alternative at hand right now... will have to think more.