Updating Docker file to use Ubuntu 18.04
this update uses ubuntu 18.04 and python3
As the latest version PyTorch requires CUDA version > 10 -- which I have modified in the Makefile
-ARG base_img=ubuntu:16.04
+ARG base_img=ubuntu:18.04
FROM ${base_img}
# Optional args
ARG cuda=0
-ARG python_version=2.7
+ARG python_version=3
ARG pyro_branch=release
ARG pytorch_branch=release
ARG uid=1000
