`--docker-image` should imply `--docker`

Either I'm missing something, or I'm doing something wrong. Both are likely.

I have two docker images, purely to build for different deployments. Primarily I don't want to build with docker, hence I have the following in `stack.yaml`.

```
docker:
enabled: false
```

When trying to locate the produced product, using `stack --docker-image target/debian8.3 path --dist-dir`, I get the exactly same result as if I had omitted the `--docker-image` argument. To get the actual path for the products, I need to specify `--docker --docker-image target/debian8.3`.

So we have:

```
stack --docker-image target/debian8.3 path --dist-dir == stack path --dist-dir
```

where I would have expected

```
stack --docker-image target/debian8.3 path --dist-dir == stack --docker --docker-image target/debian8.3 path --dist-dir
```

Should `--docker-image` not imply `--docker`?