kbld unpackage fails to honor docker config with harbor v2

## Context

```bash
$ kbld version
kbld version 0.24.0

Succeeded
```

1. We upgraded one of our harbor instances to v2 and kbld suddenly stopped working.
1. Our config was relying on the [user's docker config](https://github.com/k14s/kbld/blob/develop/docs/auth.md#via-docker-config).
1. Thankfully `v0.23.0+` supports environment variables and it looks like it honors those if they are present.

Our suspicion is that `kbld` is not honoring the credentials across redirects.

### Failure

```
kbld: Error: Importing image gcr.io/cf-networking-images/cf-k8s-networking/routecontroller@sha256:ed4b3e351a31313ebf974439e4fb43210281a02f0a9125cb8ea880c572385b5f:
Importing image as example.com/pull-requests/tas-for-kubernetes@sha256:ed4b3e351a31313ebf974439e4fb43210281a02f0a9125cb8ea880c572385b5f:
Writing image:
Retried 5 times:
HEAD https://example.com/v2/pull-requests/tas-for-kubernetes/blobs/sha256:2278c072850b5981cc98736548509e528ee6fca05bda90d75119ee953c8facf9: unsupported status code 401
```

> Note: Edited internal host for privacy.