Fix inherited env var not found in multi-var .env

[flemay]

Note: This PR is a replication of my previous PR in compose-spec/godotenv repository.

The package dotenv implements inherited environment variables which allows variables to be set on the host and passed to a container using key-only pairs in a .env file. Package compose-go uses this feature by calling the function ParseWithLookup of dotenv.

One expectation of this feature is that an environment variable will not be passed to the container if the host does not set it. This scenario is tested in dotenv. However, there is an issue if a .env file contains multiple key pairs resulting of environment variables not being set properly in the container.

This PR fixes this issue.

Demonstration

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 (V1)

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

With Docker-Compose (V1), we get the same outcome as Docker

Compose (v2)

$ 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.

[flemay]

Closing this PR because the PR is more accurate. :)