Parallelize docker dependency builds (#70866)
Summary:
Those scripts are run on 8 vCPU instances, so passing at least `-j6` makes sense
Pull Request resolved: https://github.com/pytorch/pytorch/pull/70866
Reviewed By: atalman
Differential Revision: D33435083
Pulled By: malfet
fbshipit-source-id: c879ed928da0b77346a92976d2fe9ad92ba01b5e
