DL3022 - COPY --from= when references an external image

Hi,
Just fall into this

### Expected behavior
`COPY    --from=myregistry/my/image:tag /binary /`
should be correct

### Actual behavior
Prompt a DL3022 error
`COPY --from should reference a previously defined FROM alias`

### Output of `hadolint --version`
```bash
Haskell Dockerfile Linter v1.6.5-0-g933c2e2
```
