Docker buildx does not have access to restricted repos

[klutchell]

Since we made the change in #335 to login to registries after all Docker build and tests, we no longer have the option to cache-to or cache-from registries that require authentication.

Example: https://github.com/klutchell/dnscrypt-proxy-docker/blob/d33b53b670dc76808164ef69d09c30731fecf000/docker-bake.hcl#L18-L27

We could could consider logging in before Docker bake, but we need to consider if that would potentially leak secrets during a bake step?

[klutchell]

This is also an issue when trying to use private repos as a base image.