Add example Dockerfile and documentation

[henrydavidge]

What changes are proposed in this pull request?

Earlier, we decided to remove the collection of dockerfiles in the glow repo since it was hard to keep them in sync with DBR. This PR adds an example dockerfile that users can customize for whatever dependencies they require.

I built and pushed the resulting image to https://hub.docker.com/layers/henrydavidge/glow-test/glow2/images/sha256-8894be1757191ca61cee2f75b2f05e952de5a5a7cac3c341e716483710c22c0e?context=repo.

How is this patch tested?

• [ ] Unit tests
• [ ] Integration tests
• [x] Manual tests

(Details)