Implemented checksum computation on image creation (necessary for new push primitive)

container.go

@@ -1,6 +1,7 @@
 package docker
 
 import (
+	"crypto/sha256"
 	"encoding/json"
 	"fmt"
 	"github.com/dotcloud/docker/rcli"
@@ -695,6 +696,18 @@ func (container *Container) ExportRw() (Archive, error) {
 	return Tar(container.rwPath(), Uncompressed)
 }
 
+func (container *Container) RwChecksum() (string, error) {
+	h := sha256.New()
+	rwData, err := container.ExportRw()
+	if err != nil {
+		return "", err
+	}
+	if _, err := io.Copy(h, rwData); err != nil {
+		return "", err
+	}
+	return string(h.Sum(nil)), nil
+}
+
 func (container *Container) Export() (Archive, error) {
 	if err := container.EnsureMounted(); err != nil {
 		return nil, err