docker: add environment variable PYTORCH_VERSION (#50154)
Summary:
The aim is being able to inspect a container image and determine immediately
which version of pytorch it contains.
Closes https://github.com/pytorch/pytorch/issues/48324
Signed-off-by: Felix Abecassis <fabecassis@nvidia.com>
seemethere PTAL.
As you requested in https://github.com/pytorch/pytorch/issues/48324#issuecomment-754237156, I'm submitting the patch. But I could only do limited testing as I'm not sure these Makefile/Dockerfile are used for pushing the Docker Hub images (since the Makefile tags the image with a `v` prefix for the version, as in: `pytorch:v1.7.1`, but Docker Hub images don't have this prefix).
Also on the master branch we currently have the following:
```
$ git describe --tags
v1.4.0a0-11171-g68a6e46379
```
So it's a little off, but it behaves as expected on the `release/1.7` branch.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/50154
Reviewed By: walterddr
Differential Revision: D25828491
Pulled By: seemethere
fbshipit-source-id: 500ec96cb5f5da1321610002d5e3678f4b0b94b5
