Add a Dockerfile to build a Docker image to run HandyRep effortlessly.

Assuming that one has Docker running on their machine, they can run HandyRep
within a container, by running the following commands:

    docker build -t handyrep .
    docker run -d -P -v /path/to/some/handyrep.config:/handyrep/handyrep.conf handyrep

For the record, `-d` just means "run this container in the background", and `-P` means
"expose the TCP ports of this container". You can check on which port the REST API
is listening with `docker ps`.

Woud you like me to add some explanations in one of the documentation files as well?