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>
It might be nice to have an example or explanation of when/why you would want to do this. It's kind of counter-intuitive, so an example might help clarify. 

Not a requirement, just a suggestion.