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>
Keep in mind, "include" might be a bad choice of a word given how "exclude" works.  "include" is just the list of files we start with to build the total list - in particular when it has a dir in there, while "exclude" subsets that list.  So, I guess you could think of it as "exclude" wins since it comes last.  However, we need a way to say "no matter what is in the excludes, include a,b,c".  If it makes folks feel better I can make it so that the "keep no matter what list" is in a different input arg (not 'include', or 'exclude').  Would that calm people's worries?