Add Dockerfile

[MartinEthier]

Feature

Add Dockerfile so users can setup using docker instead of installing everything directly.

Motivation

Many people would prefer to keep all of the setup inside of Docker containers especially since it requires building APEX and spconv. It would also greatly simplify the installation process.

Pitch

This repo is heavily based on the https://github.com/traveller59/second.pytorch repo which contains a Dockerfile. Their Dockerfile is almost exactly what would be needed in this repo so copying that over and modifying it should be relatively straightforward.

[poodarchu]

you can make a pr if you're interested.