Allow registry auth for local output

[crazy-max]

Atm we skip the registry auth step when a build with local output is made:

• https://github.com/docker/github-builder-experimental/blob/459aa577355d351ab493db72cd04fe13b0775695/.github/workflows/bake.yml#L678

We should support registry auth for local output as user might want to pull images from private repos.