Cannot use local container images

When trying to export CAs from a local docker image, paranoia first seems to attempt to pull the image from docker.
This makes it difficult to test a file that has just been built. A couple of examples:

```sh
paranoia export cert-manager.io/local:20230311.deb12u1.0 --output json
```

A lot of output but the error is easy to see:

```log
error: failed to load image: GET https://cert-manager.io/v2/: unexpected status code 404 Not Found:
```

I tried again with a different tag:

```sh
paranoia export trust-package-local:20230311.deb12u1.0 --output json
```

Similar error:

```log
error: failed to load image: GET https://index.docker.io/v2/library/trust-package-local/manifests/20230311.deb12u1.0: UNAUTHORIZED: authentication required; [map[Action:pull Class: Name:library/trust-package-local Type:repository]]
```

A workaround is to just push the image to ttl.sh:

```sh
docker tag cert-manager.io/local:20230311.deb12u1.0 ttl.sh/cert-manager-package-debian-dev-peter:2h
paranoia export ttl.sh/cert-manager-package-debian-dev-peter@sha256:6ab5072f567b31d469c580e2c6984cdf7289f12ff03d6415518a69c6bb817e4b --output json
```