builder: properly communicate onbuild trigger information during
subsequent builds.

Docker-DCO-1.1-Signed-off-by: Erik Hollensbe <github@hollensbe.org> (github: erikh)
Make sure COPY/ADD on dirs doesn't grab too many files
Add check for / first - per LK4D4's comment.
Add a comment to explain why we're adding a /

Signed-off-by: Doug Davis <dug@us.ibm.com>
Merge pull request #8187 from erikh/builder_onbuild_output

builder: properly communicate onbuild trigger information during subsequent builds.
Merge pull request #8186 from duglin/ErrInCopyCache

Make sure COPY/ADD on dirs doesn't grab too many files