fix: unnecessary .env file usage

[jrasm91]

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[gboudreau]

I was wondering why the env_file value was not needed.
The lines that follow (environment and volumes) use variables that are defined in the .env file, eg. POSTGRES_PASSWORD: ${DB_PASSWORD}, so if you remove env_file, how is docker-compose loading those values?

Answering myself: variables defined in env_file are NOT available as variables in the compose file; they are injected as environment variables when running the container. Variables available in the compose file are defined in the .env file, in the same folder as the compose.yml file.
Re-using .env as env_file makes sense, because that means Docker containers configurations will match values used by running containers.