Add support for running service in Docker container

- Provide Dockerfile to build and run the service in container
- Using environment variables for config
- Supporting also .env to provide config
- Updated documentation accordingly
Sorry for causing confusion with that `start-container`. Because this service cannot be started with -e parameters (unless .env file is used), it's unlikely people will use `start-container` in the end. As you pointed out, the command starts the service in shell and exists together with shell. I intended the command kinda for exploring purposes: In similar to `node server` command - one rarely runs production in that way. But maybe I should have written it as  `docker run -P -i -t bitbucket-slack-pr-hook npm start /bin/bash` (handy for development purposes, btw)

Long story short: I suggest to start container as documented in README.md while package scripts can be helpful for some.