Tag existing images in container registry

I want to add a tag for an image that already exists in the container registry.

I'd love to have a `doctl registry tag` command, similar to `docker tag`:

```
doctl registry tag image-name:old-tag image-name:new-tag
```

This way tagging can be done from the command line without downloading and then reuploading the image.

I'm assuming this will also require changes in the container registry itself.