--ignore-permissions flag to `docker build` now ignores files that cannot be accessed.

Adds an `--ignore-permissions` flag to `docker build` to ignore permissions errors while creating the context to upload.

Resolves #6521.
@erikh what's the expected functionality? when i'm using the cli tar, it doesn't add the target file but the symlink. And the symlink might not mean anything wherever it is untarred if the corresponding target is not there (or is different).

ping @crosbymichael @vieux