Fix ADD caching issue with . prefixed path

Docker-DCO-1.0-Signed-off-by: Guillaume J. Charmes <charmes.guillaume@gmail.com> (github: creack)
Make vfs error more explicit

Docker-DCO-1.0-Signed-off-by: Guillaume J. Charmes <guillaume.charmes@docker.com> (github: creack)
Fix issue with file caching + prevent wrong cache hit

Docker-DCO-1.0-Signed-off-by: Guillaume J. Charmes <guillaume.charmes@docker.com> (github: creack)