entrypoint: add docker secret friendly _FILE options

[andwei]

Hi, I was deploying the onlyoffice documentserver community edition using docker swarm and thought it would be nice to be able to use docker secrets to avoid putting JWT_SECRET or DB_PWD as environment variables into the compose file. This adds the ability to specify a file, e.g. created from a docker secret, to use as a secret (similar to what mariadb and many other containers do).

Kind regards, Andreas

[CLAassistant]

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[cirune]

Yes please. Thank you for creating this commit!