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
There are a lot of blog posts and articles about Docker security (or how it should not be considered as fully isolated). If service runs with root permissions, it's almost as having root permissions in Docker host, too. Thus, running the service as non-root user is highly recommended: https://docs.docker.com/articles/security/#conclusions