Add Dockerfile to build and run LibreTranslate as a Docker image. Also added a...

Add Dockerfile to build and run LibreTranslate as a Docker image. Also added a docker-compose.yml file to provide a reproducible way to run the Docker image. And finally added how to build and run the Docker image to the README.md. Tested with local deployment on port 5000 and public HTTPS deployment using a docker based nginx-proxy