Allow passing of variables via an argument in hubploy that would then be accessible via the Dockerfile for an image build

[felder]

CircleCI allows us to define environment variables for use with our project, but alas there's no good way that I can think of to pass those variables through hubploy to the resulting docker image build process.

In this particular case, the token needs to be made available to R when packages are installed via the image build process in order to get around the rate limiting imposed by github for unauthenticated API requests.

[ryanlovett]

For berkeley-dsep-infra/datahub#1048.