Add a Dockerfile and fix relative path

[mdp]

• Adding a Dockerfile for easier server setup
  • Handles all the dependencies
  • Can be pulled directly from Dockerhub or built locally
• Fix path require. This breaks the docker build because the original require looks up the path to ./server which impies that you're running it out of the repo. In the case of Docker, the server code is running in it's own directory, which won't be named 'server'

Example of running docker: docker run -p 53:53/udp -it --rm mpercival/dnscat2 ruby ./dnscat2.rb foo.org

or building from source

cd dnscat2/server
docker build -t dnscat2 .
docker run -p 53:53/udp -it --rm dnscat2 ruby ./dnscat2.rb foo.org

[mdp]

This would also simplify #78