Fix env_file variable parsing on Compose file

[nvsofts]

In env_file, the current Container-Compose implemantion only supports list syntax.
Therefore it rejects following Compose.

services:
  app:
    image: alpine:latest
    env_file: app.env

According to The Compose Specification, env_file supports both string and list syntax.
https://github.com/compose-spec/compose-spec/blob/main/spec.md#env_file

This PR fixes env_file parsing in Compose file.