stack --nix image container does not generate an image

### General summary/comments (optional)

```bash
stack --nix image container --image docker.img
```
does not generate a file `docker.img`.

### Steps to reproduce

1. Create a stack project with a file `stack.yaml`
2. Run command `stack --nix -v image container --image docker.img`.
3. Observe that the program runs and exits with status 0 and no error message.
4. Observe that there is no file `docker.img`.

Include any `.yaml` configuration if relevant.

`stack.yaml` does not contain a `image:` section.

### Expected

A docker image named `docker.img`.

### Actual

No docker image.

### Stack version

```
$ stack --version
Version 1.4.0 x86_64
```

### Method of installation

* NixOS