--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.
ok but you're still returning too early if there's an error instead of going to the `addTarFile` call. Am I missing something? Also I would only do so if `IsPermission(err)` is true.