--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.
if `filePath` is not a directory, since filepath.SkipDir != nil the Walk function will halt here: https://code.google.com/p/go/source/browse/src/pkg/path/filepath/path.go?name=release#348
You should check if filePath is a dir before sending SkipDir.