cannot reuse .env in expectedOutput

My images are built by docker compose based on ARGs which are taken values from `.env`.

I would like to make the assertion accordingly:

**.env**

```env
NODE_TAG=11.8.0
```

**config.yml**
```yml
schemaVersion: '2.0.0'

globalEnvVars:
- key: "NODE_TAG"
value: "$NODE_TAG"
commandTests:
- name: "node version"
command: "node"
args: ["-v"]
expectedOutput: ["$NODE_TAG"]
```

When I ran tests, I got:

**stdout**
```
--- FAIL
stdout: v11.8.0
Error: Expected string '$NODE_TAG' not found in output 'v11.8.0
```