Docker: Use of alpine, pass in env vars and use pm2-runtime

[jleeh]

Also:

• Remove of envs completely from the app.json file. This is due to them having precedent over passed in environment variables and defaults are already set in lib/node.js
• Use of the emit instance function for sending the hello/login message so errors are shown on failure and the message can be traced in the logs
• Readme changes to better instruct any users

Docker refactor wins:

• With the use of the node alpine image, size is cut down from 278MB to 53MB.
• Using pm2-runtime with no daemon mode allows the entrypoint to be a pm2 command which stops the container if the client quits, rather than forever running just with tail -f even if the client exits.

Please also rectify #2 so we can get the correct automated Docker hub builds.