Add manual testing utilities

Created by: matthew-d-jones

Closes #69 (closed) Fixes #72 (closed)

Added a docker-compose script which can be used to create an environment for testing the writer. See the added documentation for more details.

The container is currently based on Ubuntu, which made it easy to write the dockerfile. If we end up deploying containers it would be possible to cut down the image size a little by using Alpine, otherwise if we deploy to bare metal, basing it on CentOS would be preferable.

An image for the writer is on dockerhub: https://hub.docker.com/r/screamingudder/kafka-to-nexus/

Issue #72 (closed) was found by running the writer in a container and so has also been fixed in these changes.