pull: don't resolve short names on explicit docker:// reference

Fixes: containers#581
Signed-off-by: Valentin Rothberg <rothberg@redhat.com>

libimage/pull.go

@@ -52,6 +52,7 @@ func (r *Runtime) Pull(ctx context.Context, name string, pullPolicy config.PullP
 		options = &PullOptions{}
 	}
 
+	var possiblyUnqualifiedName string // used for short-name resolution
 	ref, err := alltransports.ParseImageName(name)
 	if err != nil {
 		// If the image clearly refers to a local one, we can look it up directly.
@@ -74,6 +75,17 @@ func (r *Runtime) Pull(ctx context.Context, name string, pullPolicy config.PullP
 			return nil, err
 		}
 		ref = dockerRef
+		possiblyUnqualifiedName = name
+	} else if ref.Transport().Name() == registryTransport.Transport.Name() {
+		// Normalize the input if we're referring to the docker
+		// transport directly. That makes sure that a `docker://fedora`
+		// will resolve directly to `docker.io/library/fedora:latest`
+		// and not be subject to short-name resolution.
+		named := ref.DockerReference()
+		if named == nil {
+			return nil, errors.New("internal error: unexpected nil reference")
+		}
+		possiblyUnqualifiedName = named.String()
 	}
 
 	if options.AllTags && ref.Transport().Name() != registryTransport.Transport.Name() {
@@ -94,7 +106,7 @@ func (r *Runtime) Pull(ctx context.Context, name string, pullPolicy config.PullP
 
 	// DOCKER REGISTRY
 	case registryTransport.Transport.Name():
-		pulledImages, pullError = r.copyFromRegistry(ctx, ref, strings.TrimPrefix(name, "docker://"), pullPolicy, options)
+		pulledImages, pullError = r.copyFromRegistry(ctx, ref, possiblyUnqualifiedName, pullPolicy, options)
 
 	// DOCKER ARCHIVE
 	case dockerArchiveTransport.Transport.Name():