Define an environment variable for a service in `docker-compose.yml`

Question

How would you define an environment variable for a service in `docker-compose.yml`?

Accepted Answer

```yaml
services:
  web:
    image: "nginx:latest"
    environment:
      MY_VAR: value
```

Answer

```yaml
services:
  web:
    image: "nginx:latest"
    set:
      - MY_VAR=value
```

Answer

```yaml
services:
  web:
    image: "nginx:latest"
    variables:
      MY_VAR: value
```

Answer

```yaml
services:
  web:
    image: "nginx:latest"
    env:
      - MY_VAR=value
```

Question from the Docker compose test