updated Dockerfile

[heywoodlh]

This PR just updates the Ruby version, makes sure the Dockerfile is reproducible and sets a default entrypoint so the server always runs with the image. The intention with this PR is that the built image (in this example, I tagged it heywoodlh/dnscat2) could be run with a command like this:

docker run -it --rm -p 53:53 heywoodlh/dnscat2