respect dockerignore

As @sirreal discovered, `.dockerignore` files are currently ignored by the tar stream process in the builder. We should respect `.dockerignore` files when building the build context

Start at the builder's tar stream process and trace how the build context is assembled. Check how a .dockerignore file should affect the files sent in that context; done means ignored paths are excluded while building and other context files remain available.

Written by the indexing model from the issue text.