Maintain old API for backwards compatibility. (#1940)

Turns out rules_k8s also depended on the ReadImage function. If anyone
else is depending on this, we can maintain backwards compatibility by
keeping the same function signature and having it wrap
Reader.ReadImage().

container/go/cmd/digester/digester.go

@@ -49,8 +49,7 @@ func main() {
 	if err != nil {
 		log.Fatalf("Unable to determine parts of the image from the specified arguments: %v", err)
 	}
-	r := compat.Reader{Parts: imgParts}
-	img, err := r.ReadImage()
+	img, err := compat.ReadImage(imgParts)
 	if err != nil {
 		log.Fatalf("Error reading image: %v", err)
 	}

container/go/cmd/flattener/flattener.go

@@ -52,8 +52,7 @@ func main() {
 	if err != nil {
 		log.Fatalf("Unable to determine parts of the image from the specified arguments: %v", err)
 	}
-	r := compat.Reader{Parts: imgParts}
-	img, err := r.ReadImage()
+	img, err := compat.ReadImage(imgParts)
 	if err != nil {
 		log.Fatalf("Error reading image: %v", err)
 	}

container/go/cmd/pusher/pusher.go

@@ -99,8 +99,7 @@ func main() {
 	if err != nil {
 		log.Fatalf("Unable to determine parts of the image from the specified arguments: %v", err)
 	}
-	r := compat.Reader{Parts: imgParts}
-	img, err := r.ReadImage()
+	img, err := compat.ReadImage(imgParts)
 	if err != nil {
 		log.Fatalf("Error reading image: %v", err)
 	}

container/go/pkg/compat/reader.go

@@ -296,7 +296,7 @@ func (r *Reader) loadLayers() error {
 	return nil
 }
 
-// ReadImage loads a v1.Image from the given ImageParts
+// ReadImage loads a v1.Image from the ImageParts section in the reader.
 func (r *Reader) ReadImage() (v1.Image, error) {
 	// Special case: if we only have a tarball, we can instantiate the image
 	// directly from that. Otherwise, we'll process the image layers
@@ -343,3 +343,10 @@ func (r *Reader) ReadImage() (v1.Image, error) {
 	}
 	return img, nil
 }
+
+// ReadImage loads a v1.Image from the given ImageParts
+func ReadImage(parts ImageParts) (v1.Image, error) {
+	r := Reader{Parts: parts}
+	img, err := r.ReadImage()
+	return img, err
+}