Add support for VAR_FILE to enable use of secrets

[JCPvanHooren]

Description of your request
Enable setting environment variables from files instead of strings to enable the use of secrets.

Describe the solution you'd like
Support the VAR_FILE convention.

Describe alternatives you've considered
I've tried to run my docker compose with

entrypoint: >
      sh -c '
      export DB_PASSWORD="$(cat /run/secrets/db_pwd)";
      exec docker-php-entrypoint apache2-foreground
      '

command: >
      sh -c '
        export DB_PASSWORD="$(cat /run/secrets/db_pwd)";
        exec "$@"
      '

command:
      - sh
      - -c
      - |
        export DB_PASSWORD="$(cat /run/secrets/db_pwd)"
        exec apache2-foreground

Additional context
Apologies if this is due to my limited knowledge of Docker. If there's already a way to use secrets, I'm happy to take any recommendations and close the request.