Automated test of the docker build

[paulmelnikow]

@chris48s wrote at #2814 (comment):

I think we should be able to just call docker run inside Circle CI. That would at least give a us a smoke test that it doesn't throw an error.

A couple references:

• https://circleci.com/docs/2.0/building-docker-images/
• https://circleci.com/blog/how-to-build-a-docker-image-on-circleci-2-0/

I think ideally what we want is three things:

1. Check that container builds
2. Check that container runs
3. Fetch a badge from the server and ensure it works

We also need a way to shut down the container after 2 or 3 is finished.

[calebcartwright]

On the third step, what would a test look like? Could we simply use the badge-cli script (https://github.com/badges/shields/blob/master/scripts/badge-cli.js) or should we set up a new mocha test suite?

Also I'm guessing we'd want to include this both on PR builds as well as the daily test runs. Any disagreements there?

[paulmelnikow]

I mentioned at #3558 (comment) that a Docker Hub's automated tests might be a decent and simple way of accomplishing this.