Use Docker Secrets for Credentials

Please add an option to read the admin and user passwords from a file.

Using a file's contents allows for integration with Docker Secret Store, which enables deterministic credentials for administration purposes instead of a random password generated at container init, or worse, plaintext passwords within Docker Stack definitions.

Common convention is to use separate ENV variables suffixed with `_FILE` to define the path of files containing the information otherwise passed as plain text ENV variables.

https://github.com/influxdata/influxdata-docker/blob/56529df4a46eaac4a83c87d9f36f3ba328b4676a/influxdb/1.7/init-influxdb.sh#L21

Start with init-influxdb.sh at the referenced line, then trace how the admin and user passwords are read during container initialization. Add the requested file-based credential option using the documented _FILE convention, and verify that credentials supplied through Docker Secret Store files are accepted for both users.

Written by the indexing model from the issue text.