Fix .dockerignore when ignoring unreadable dirs

The initial `ValidateContextDirectory` implementation fails loudly when a file
lacks read permissions in the current context. However that situation is valid
if the file is included in the `.dockerignore` patterns.

Docker-DCO-1.1-Signed-off-by: Bruno Renié <brutasse@gmail.com> (github: brutasse)
Return error for utils.Matches in archive
Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@docker.com> (github: crosbymichael)
Merge pull request #7357 from crosbymichael/docker-ignore

Fix .dockerignore when ignoring unreadable dirs