--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.
I can add the os.IsPermission check, but the point for `--ignore-permissions` is to avoid adding it to the tarfile. If it is attempted, `tw.Close()` below will fail as it's expecting more in the tarfile than was added.

It still fails if `--ignore-permissions` is not set.