set up CI for docker-doom

[mccarrmb]

Dockerfile should be able to pull in an image, build the environment, and then launch a shareware server. Supported host OS will only be Linux.

CI steps:

• Run docker build
• Run container
• Query forwarded address to verify dedicated server is running
  • Will involve a Diffie-Huffman library and some scripting
• Connect to server via rlogin
  • Requires external zandronum-server executable... I think.