Add support for storing API_SECRET as a secret when running in a container

[swebster]

It should be possible to configure Nightscout to read its API_SECRET from a file so it can be injected into a host container securely without exposing it in the runtime environment. The typical solution (e.g. as used in postgres) would be to add support for a variable named API_SECRET_FILE, that could then be used to configure Nightscout to load the API key from a Docker secret stored in /run/secrets/nightscout_api_secret or similar.