Fix `docker rmi` tab completion

This is exactly what happens on tab completion for docker rmi. This commit
fixes that.

```sh
$ docker rmi <hit TAB>
_arguments:comparguments:312: invalid argument: __docker_images
_arguments:comparguments:312: invalid argument: __docker_images
_arguments:comparguments:312: invalid argument: __docker_images
```
Merge pull request #2693 from javiertejero/master

Fix `docker rmi` tab completion