Log based on registry known-support - reduce noise on notifications (#716)

Log based on registry known-poor support of HEAD in checking container
manifest.

Some private registries do not support HEAD (E.G. GitLab Container Registry).
With the current config, this log message is causing a notification to be
sent for each container hosted in a registry lacking HEAD support.

log.Debug or log.Warning for failed HTTP HEAD-check based on registry hostname
where HEAD-check is known to fail.

For Docker Hub, a failed HEAD leading to a "regular pull" may count against a
user's call-quota whereas other registry implementations do not support HEAD,
or whose container manifest may be in a different location.

pkg/container/client.go

@@ -295,7 +295,11 @@ func (client dockerClient) PullImage(ctx context.Context, container Container) e
 	log.WithFields(fields).Debugf("Checking if pull is needed")
 
 	if match, err := digest.CompareDigest(container, opts.RegistryAuth); err != nil {
-		log.Info("Could not do a head request, falling back to regular pull.")
+		if registry.WarnOnAPIConsumption(container) {
+			log.WithFields(fields).Warning("Could not do a head request, falling back to regular pull.")
+		} else {
+			log.Debug("Could not do a head request, falling back to regular pull.")
+		}
 		log.Debugf("Reason: %s", err.Error())
 	} else if match {
 		log.Debug("No pull needed. Skipping image.")

pkg/registry/registry.go

@@ -1,6 +1,9 @@
 package registry
 
 import (
+	"github.com/containrrr/watchtower/pkg/registry/helpers"
+	watchtowerTypes "github.com/containrrr/watchtower/pkg/types"
+	ref "github.com/docker/distribution/reference"
 	"github.com/docker/docker/api/types"
 	log "github.com/sirupsen/logrus"
 )
@@ -31,3 +34,26 @@ func DefaultAuthHandler() (string, error) {
 	log.Debug("Authentication request was rejected. Trying again without authentication")
 	return "", nil
 }
+
+// WarnOnAPIConsumption will return true if the registry is known-expected
+// to respond well to HTTP HEAD in checking the container digest -- or if there
+// are problems parsing the container hostname.
+// Will return false if behavior for container is unknown.
+func WarnOnAPIConsumption(container watchtowerTypes.Container) bool {
+
+	normalizedName, err := ref.ParseNormalizedNamed(container.ImageName())
+	if err != nil {
+		return true
+	}
+
+	containerHost, err := helpers.NormalizeRegistry(normalizedName.String())
+	if err != nil {
+		return true
+	}
+
+	if containerHost == "index.docker.io" || containerHost == "ghcr.io" {
+		return true
+	}
+
+	return false
+}