Add support for _FILE environment variables so Docker secrets can be used for credentials

[ghost]

Would you consider adding in support for specifying environment variables with the standardised _FILE appended which are then read from a secret file and converted to an environment variable? This would allow a more secure way of setting the credentials used by the VPN & SOCKS clients.

More info: https://docs.docker.com/engine/swarm/secrets/#build-support-for-docker-secrets-into-your-images

Also: yobasystems/alpine-mariadb#32