Support env var resolution before substitution inside .env about compose-go HOT 2 CLOSED

Thank you @milas

Actually I had tried with a buggy version Docker Compose version v2.0.1 which resulted in:

$ docker compose run --rm alpine env | grep SUB_DOMAIN                                     
SUB_DOMAIN=a.:-foo.bar}

Now working with both 1.29.2 and 2.11.2.

from compose-go.

We are working on formalizing and documenting the environment variable interpolation rules/syntax.

If you want the shell to take precedence, try the following:

MAIN_DOMAIN="${MAIN_DOMAIN:-foo.bar}"

from compose-go.