Support _FILE environment variables in Docker Compose

[daniel-mader]

Description

Allow supplying a file that contains a secret instead of supplying the value directly in the docker-compose.yaml as described in https://docs.docker.com/compose/use-secrets/.

Motivation

We want to support that convention to further enhance the usability and flexibility of the service.

Requirements

• add parsing logic for env variables that contain a secret and that have the _FILE suffix
• rename docker-compose.yml to compose.yaml to follow the latest Docker spec

Open Questions

• How do we treat multiple values for a single secret in a file? (Only take first line? Fail?)
• Is it possible to have multiple secrets in a single file? Or does each secret need it's own file?

Are you planning to contribute this in a PR?

Yes