Support inherited environment variable not found with multiple vars

[flemay]

Hi!

I have been testing the command docker compose with environment variable files and the result differs from Docker and Docker-Compose.

TL;DR

Inherited environment variable not found is already implemented but does not work when there are other env vars defined. This PR fixes it. :)

Test

Define a .env file with the following:

ENV_A
ENV_B=
ENV_C=env_c

And a docker-compose.yml file:

services:
  alpine:
    image: alpine
    env_file: .env

Docker

$ docker run --rm --env-file=.env alpine env
ENV_B=
ENV_C=env_c

Here ENV_A has not been set and ENV_B is set with an empty value.

Docker-Compose

$ docker-compose run --rm alpine env
ENV_B=
ENV_C=env_c

With Docker-Compose, we get the same outcome as Docker

Compose

$ docker compose run --rm alpine env
ENV_C=env_c
ENV_A=ENV_B=

Here's where the outcome is quite unexpected: ENV_A is set with the value ENV_B=.

Compose seems to have a problem with key-only items in .env file.

Fix

After investigation, I ended up finding the issue with godotenv which is used by compose-go and docker compose. So I decided to create this PR to fix the compatibility issue.

[ndeloof]

Hi.
We have merged godotenv into compose-go by compose-spec/compose-go#215
Could you please apply this pull-request to this repository?