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
In `package.json` you used `-P` for starting the container. Shouldn't we be cohesive here? I'd vote for `-p 5000:5000` approach (not exposing more than we need to). What do you think?