Support alternative syntax for defining default values for environment variables

[DeepDiver1975]

Would it be possible to support an alternative syntax for defining default values for env vars?

To be more precise supporting docker compose syntax is what I have in mind: https://docs.docker.com/compose/environment-variables/env-file/#interpolation

THX a lot

(Happy to contribute in case this is of interest)

[inhere]

related https://github.com/gookit/config/issues/157