In Dockerfile, do not install unecessary packages, use conda to install ninja (saving one layer), and use "." to refer to WORKDIR to reduce redundancy. (#20881)
Summary:
- Do not install unecessary packages in the Docker image.
- In the Docker image, use conda to install ninja (saving one layer)
- When workdir is set, use "." to refer to it to reduce redundancy.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/20881
Differential Revision: D15495769
Pulled By: ezyang
fbshipit-source-id: dab7df71ac107c85fb1447697e25978daffc7e0b
