Can I use a file name other than `.env` for interpolation?

Yes. By default, Compose looks for exactly `.env`. If you want to use `.env.production` for interpolation, you must run it like this: `docker-compose --env-file .env.production up -d`.

What if I use interpolation `${VAR}` but the variable doesn't exist in the `.env` file?

Compose will silently substitute an empty string (nothing). This can cause catastrophic bugs if your app expects a value. You can enforce a default fallback value in YAML using `${VAR:-default_value}`. If VAR is missing, it uses 'default_value'.

Can I inspect the final variables inside the running container?

Yes. You can run `docker exec -it env`. This will print out the final, resolved list of every single environment variable currently active inside that specific container.

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Environment Configuration

Master Environment Variable management in Docker Compose. Learn the critical difference between the `environment:` and `env_file:` directives, and understand how to securely interpolate secrets using hidden `.env` files.

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