image.requires.alias is not available when using a custom buildx

### Expected behavior
I expect the image alias to be available in the Dockerfile when using custom-buildx

### Actual behavior

A custom [base image variable](https://github.com/GoogleContainerTools/skaffold/issues/889#issuecomment-736718801
) cannot be accessed from a Dockerfile in the [custom-buildx example](https://github.com/GoogleContainerTools/skaffold/tree/main/examples/custom-buildx)

### Information

- Skaffold version: v1.37.0
- Installed via: Homebrew