Kubelet: Use RepoDigest for ImageID when available

Previously, we used the docker config digest (also called "image ID"
by Docker) for the value of the `ImageID` field in the container status.
This was not particularly useful, since the config manifest is not
what's used to identify the image in a registry, which uses the manifest
digest instead.  Docker 1.12+ always populates the RepoDigests field
with the manifest digests, and Docker 1.10 and 1.11 populate it when
images are pulled by digest.

This commit changes `ImageID` to point to the the manifest digest when
available, using the prefix `docker-pullable://` (instead of
`docker://`)

test/e2e_node/image_id_test.go

@@ -0,0 +1,66 @@
+/*
+Copyright 2016 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package e2e_node
+
+import (
+	"k8s.io/kubernetes/pkg/api"
+	"k8s.io/kubernetes/pkg/kubelet/dockertools"
+	"k8s.io/kubernetes/test/e2e/framework"
+
+	"github.com/davecgh/go-spew/spew"
+	. "github.com/onsi/ginkgo"
+	. "github.com/onsi/gomega"
+)
+
+var _ = framework.KubeDescribe("ImageID", func() {
+
+	busyBoxImage := "gcr.io/google_containers/busybox@sha256:4bdd623e848417d96127e16037743f0cd8b528c026e9175e22a84f639eca58ff"
+
+	f := framework.NewDefaultFramework("image-id-test")
+
+	It("should be set to the manifest digest (from RepoDigests) when available", func() {
+		podDesc := &api.Pod{
+			ObjectMeta: api.ObjectMeta{
+				Name: "pod-with-repodigest",
+			},
+			Spec: api.PodSpec{
+				Containers: []api.Container{{
+					Name:    "test",
+					Image:   busyBoxImage,
+					Command: []string{"sh"},
+				}},
+				RestartPolicy: api.RestartPolicyNever,
+			},
+		}
+
+		pod := f.PodClient().Create(podDesc)
+
+		framework.ExpectNoError(framework.WaitTimeoutForPodNoLongerRunningInNamespace(
+			f.Client, pod.Name, f.Namespace.Name, "", framework.PodStartTimeout))
+		runningPod, err := f.PodClient().Get(pod.Name)
+		framework.ExpectNoError(err)
+
+		status := runningPod.Status
+
+		if len(status.ContainerStatuses) == 0 {
+			framework.Failf("Unexpected pod status; %s", spew.Sdump(status))
+			return
+		}
+
+		Expect(status.ContainerStatuses[0].ImageID).To(Equal(dockertools.DockerPullablePrefix + busyBoxImage))
+	})
+})