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>
for thoroughness, I would not return here. One of the reasons we moved to this new structure for the sums list, is because a valid tar archive could have multiple files of the same path (they would clobber each other).