Add Docker quickstart to README

[abrichr]

The instructions in the existing quickstart section in the README are less deterministic and more complex than the Dockerfile equivalent:

# Build the image
sudo nvidia-docker build -t som .

# Run the image
sudo docker run -d -p 6092:6092 --gpus all --name som-container som

Consider prepending or replacing the existing quickstart section with the above Dockerfile quickstart.