imagetools create should support copying images between repositories

It's [currently not possible to create a new image based on source images with multiple repositories](https://github.com/docker/buildx/blob/d1a46faf8475564fbb27c9228b79a806e59b8f65/commands/imagetools/create.go#L79):

```
multiple repositories currently not supported, found map[docker.io/crazymax/diun:{} localhost:5000/name/app:{}]
```