Global defaults for image configs

Allows setting of image configs at a global level to act as default
values.

This required a change in the model.Image struct due to a bool field not
having a third, unset state. The remedy is to unmarshal into a temporary
data structure to detect the presents of a field value and then use that
to determine if the default value should be used.

Fixes #491

internal/config/config_test.go

@@ -7,6 +7,7 @@ import (
 
 	"github.com/crazy-max/diun/v4/internal/config"
 	"github.com/crazy-max/diun/v4/internal/model"
+	"github.com/crazy-max/diun/v4/pkg/registry"
 	"github.com/crazy-max/diun/v4/pkg/utl"
 	"github.com/crazy-max/gonfig/env"
 	"github.com/stretchr/testify/assert"
@@ -58,6 +59,10 @@ func TestLoadFile(t *testing.T) {
 						BaseURL: "https://hc-ping.com/",
 						UUID:    "5bf66975-d4c7-4bf5-bcc8-b8d8a82ea278",
 					},
+					ImageDefaults: &model.Image{
+						NotifyOn: model.NotifyOnDefaults,
+						SortTags: registry.SortTagReverse,
+					},
 				},
 				Notif: &model.Notif{
 					Amqp: &model.NotifAmqp{