Update Docker file section in README.md (pytorch#16812)

Summary:
Emphasize on the fact that docker build should be triggered from pytorch repo directory.
Pull Request resolved: pytorch#16812

Differential Revision: D13985531

Pulled By: soumith

fbshipit-source-id: c6511d1e81476eb795b37fb0ad23e8951dbca617

Author: mohanasra

README.md

@@ -199,7 +199,7 @@ python setup.py install
 
 ### Docker Image
 
-Dockerfile is supplied to build images with cuda support and cudnn v7. You can pass `-e PYTHON_VERSION=x.y` flag to specify which Python version is to be used by Miniconda, or leave it unset to use the default. Build as usual
+Dockerfile is supplied to build images with cuda support and cudnn v7. You can pass `-e PYTHON_VERSION=x.y` flag to specify which Python version is to be used by Miniconda, or leave it unset to use the default. Build from pytorch repo directory as docker needs to copy git repo into docker filesystem while building the image.
 ```
 docker build -t pytorch -f docker/pytorch/Dockerfile .
 ```