provide docker image for running goat-guardian

[cdepillabout]

As long as you have stack, goat-guardian should be easy to install.

However, it would be nice to provide a docker image for running goat-guardian. This should be similar to the docker image for running login-with: https://github.com/lipp/login-with#deployment-with-docker

(If #14 gets implemented, it is debatable as to whether this docker image should also contain PostgreSQL. I imagine it probably should not contain PostgreSQL. The operator can install a separate PostgreSQL docker image if they want to use the PostgreSQL functionality.)