Re-introduce docker support

[badgerwithagun]

Back when the architecture was more microservicey, Docker was the only simple way to deploy the application. This was abandoned as being unnecessarily complex for the use case, but Docker support was removed at the same time (since there was no driver for docker support for the single-app build at the time).

Docker support should be re-introduced and published to Docker Hub with each release, suitably documented and configurable. A simple alpine+openjdk8 image should be quite sufficient.

It would be sensible, having done so, to funnel Heroku deployment via Docker, to ensure consistent architecture. This would also ensure that Docker and Heroku users will be cross-testing.

More info here:

https://devcenter.heroku.com/categories/deploying-with-docker

There is some crossover with #115 here. Do we build the docker image directly from source or from an installer? The latter would have the advantage, again, of making sure installer-based builds are being regularly tested without any additional effort.