Thanks for the Dockerfile.
After adding RUN apt-get install curl and the install command for git, I was able to build PyTorch as well as torchvision without any errors:
root@9c9d62ffdfcf:/# python
Python 3.6.10 |Anaconda, Inc.| (default, May  8 2020, 02:54:21)
[GCC 7.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import torch
>>> torch.__version__
'1.8.0a0+4ab73c1'
>>> import torchvision
>>> torchvision.__version__
'0.8.0a0+217e26f'
>>>