Modify Dockerfile to use included `sirv` server instead of `nginx`

The included sirv server is lightweight and production-ready. The Dockerfile should be updated to to use included sirv server instead of nginx.

The server can be started by make start.

Advantages:

• The make start command does a build of the JavaScript bundle before the server is actually started. This means changes in the .env file are updated in the app. This allows more flexibilty and omits the need of seding generated bundle files
• No NGINX container required, image can be even smaller