Update Docker file section in README.md (#16812)
Summary:
Emphasize on the fact that docker build should be triggered from pytorch repo directory.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16812
Differential Revision: D13985531
Pulled By: soumith
fbshipit-source-id: c6511d1e81476eb795b37fb0ad23e8951dbca617
