feat: pass registry auth option to imagePull func

[wybczu]

This will enable users to use authed docker registries. An example for Google Cloud Artifact Registry:

func Example_artifact_registry_auth() {
	dockerImageName := "us-docker.pkg.dev/GOOGLE_CLOUD_PROJECT/REGISTRY/image:latest"

	ctx := context.Background()

	// using github.com/google/go-containerregistry/pkg/v1/google
	authenticator, err := google.NewEnvAuthenticator(ctx)
	if err != nil {
		panic(err)
	}

	authConfig, err := authenticator.Authorization()
	if err != nil {
		panic(err)
	}

	authConfigJSON, err := authConfig.MarshalJSON()
	if err != nil {
		panic(err)
	}

	opts := dktest.Options{
		PortRequired:     true,
		ReadyFunc:        func(ctx context.Context, c dktest.ContainerInfo) bool { return true },
		LogStderr:        true,
		LogStdout:        true,
		PullRegistryAuth: base64.StdEncoding.EncodeToString(authConfigJSON),
	}

	dktest.Run(&testing.T{}, dockerImageName, opts, func(t *testing.T, c dktest.ContainerInfo) {})

	// Output:
}

[dhui]

Thanks for the fix @wybczu!