docker compose does not handle variable defaults in --env-file (docker-compose does)

[jj-meyer]

Description
Variables defaults declared in the --env-file are not passed to a compose-file.

$ docker compose version
    Docker Compose version 2.0.0-beta.5

$ docker-compose --version
    docker-compose version 1.29.2

Steps to reproduce the issue:

1. Create ./foo.yml compose-file in the project-directory

services:
  nginx:
    build:
      context: .
      args:
        - FOO=${FOO}
    environment:
        - BAR=${BAR}

2. Create ./foo.env environment file

FOO="${FOO:-FOO_ENV_DEFAULT}"
BAR="${BAR:-BAR_ENV_DEFAULT}"

The Project folder should contain ...

./foo.yml
./foo.env

Describe the results you received:

1. docker compose fails - FOO and BAR defaults are not substituted correctly when passed into foo.yml

$ docker compose --project-directory . --env-file foo.env --file foo.yml config

...

services:
  nginx:
      args:
        FOO: :-FOO_ENV_DEFAULT} ❌
    environment:
      BAR: :-BAR_ENV_DEFAULT} ❌

2. docker compose with foo.yml and declared environment variables succeeds

$ FOO="FOO_ENV_CMDLINE" BAR="BAR_ENV_CMDLINE" docker compose --project-directory . --env-file foo.env --file foo.yml config

...

services:
  nginx:
      args:
        FOO: FOO_ENV_CMDLINE ✅
    environment:
      BAR: :-BAR_ENV_CMDLINE ✅

3. docker-compose succeeds - FOO and BAR variables are passed correctly

$ docker-compose --project-directory . --env-file foo.env --file foo.yml config

...

services:
  nginx:
    build:
      args:
        FOO: FOO_ENV_DEFAULT ✅
    environment:
      BAR: BAR_ENV_DEFAULT ✅

Describe the results you expected:
Variable defaults declared in the --env-file should be passed to a compose-file.

This works correctly when the environment variables are declared and when docker-compose (Python version) is used.

Additional information you deem important (e.g. issue happens only occasionally):

Output of docker version:

$ docker --version
    Docker version 20.10.7, build f0df350

$ docker compose version
    Docker Compose version 2.0.0-beta.5

$ docker-compose --version
    docker-compose version 1.29.2, build 5becea4c

[jj-meyer]

Although not the same result, it may be that #1892 can be related to the same .env variable handling issue.

[ndeloof]

root cause is that the godotenv library we use to parse .env file does not support the ${*:-*} syntax for default variable values.

Would need to improve https://github.com/joho/godotenv/blob/ddf83eb33bbb136f62617a409142b74b91dbcff3/godotenv.go#L330

[ndeloof]

internally tracked as https://docker.atlassian.net/browse/IL-393

[charlesritchea]

I didn't see an already open ticket for godotenv so I created one joho/godotenv#153

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[phillipuniverse]

This is not stale and is still an active issue in V2 of Docker Compose.

[stale]

This issue has been automatically marked as not stale anymore due to the recent activity.

[ulyssessouza]

Should be fixed by compose-spec/compose-go#276 and included in the next release of Docker Compose.