Add docs for --env-file option.

[DavidTheProgrammer]

File: compose/environment-variables.md

Add documentation for --env-file flag merged in docker/compose#6535

[usha-mandya]

Closing this issue as it has been fixed - https://docs.docker.com/compose/environment-variables/#substitute-environment-variables-in-compose-files.

[docker-robott]

Closed issues are locked after 30 days of inactivity.
This helps our team focus on active issues.

If you have found a problem that seems similar to this, please open a new issue.

/lifecycle locked