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/provider/docker/container.go

@@ -91,7 +91,7 @@ func (c *Client) listContainerImage() []model.Image {
 			Str("ctn_image", imageName).
 			Interface("ctn_labels", ctn.Labels).
 			Msg("Validate image")
-		image, err := provider.ValidateImage(imageName, metadata(ctn), ctn.Labels, *c.config.WatchByDefault)
+		image, err := provider.ValidateImage(imageName, metadata(ctn), ctn.Labels, *c.config.WatchByDefault, *c.imageDefaults)
 
 		if err != nil {
 			c.logger.Error().Err(err).

internal/provider/docker/docker.go

@@ -10,16 +10,18 @@ import (
 // Client represents an active docker provider object
 type Client struct {
 	*provider.Client
-	config *model.PrdDocker
-	logger zerolog.Logger
+	config        *model.PrdDocker
+	logger        zerolog.Logger
+	imageDefaults *model.Image
 }
 
 // New creates new docker provider instance
-func New(config *model.PrdDocker) *provider.Client {
+func New(config *model.PrdDocker, imageDefaults *model.Image) *provider.Client {
 	return &provider.Client{
 		Handler: &Client{
-			config: config,
-			logger: log.With().Str("provider", "docker").Logger(),
+			config:        config,
+			logger:        log.With().Str("provider", "docker").Logger(),
+			imageDefaults: imageDefaults,
 		},
 	}
 }