"image status should support all kinds of references" is failing for docker runtime

[feiskyer]

The test case "image status should support all kinds of references" added in #468 is failing constantly:

[k8s.io] Image Manager
/home/travis/gopath/src/github.com/kubernetes-sigs/cri-tools/pkg/framework/framework.go:72
  image status should support all kinds of references [Conformance] [AfterEach]
  /home/travis/gopath/src/github.com/kubernetes-sigs/cri-tools/pkg/validate/image.go:60
  image status with "sha256:1d42c7758256ea94f23224403e71455e512de249b689426a5516c56e9446207f"
  Expected
      <*v1alpha2.Image | 0x0>: nil
  to equal
      <*v1alpha2.Image | 0xc0001d2fc0>: {
          Id: "sha256:1d42c7758256ea94f23224403e71455e512de249b689426a5516c56e9446207f",
          RepoTags: [
              "gcr.io/cri-tools/test-image-tag:test",
          ],
          RepoDigests: [
              "gcr.io/cri-tools/test-image-tag@sha256:e43af008538974be41b4a7fe849f83acc0e348e13ca30d1c9b8fddd8d20a70b2",
          ],
          Size_: 1146369,
          Uid: nil,
          Username: "",
      }

From kubelet logs, it's because dockershim refuses the image inspect requests for digest-only references:

I0614 14:59:38.222874    1321 helpers.go:124] Inspected image ("sha256:1d42c7758256ea94f23224403e71455e512de249b689426a5516c56e9446207f") does not match sha256:1d42c7758256ea94f23224403e71455e512de249b689426a5516c56e9446207f

@Random-Liu @yujuhong

[feiskyer]

Added a workaround in #475, need more investigation whether it should be fixed in dockershim.