Automatic merge from submit-queue
Kubelet: Use RepoDigest for ImageID when available
**Release note**:
```release-note
Use manifest digest (as `docker-pullable://`) as ImageID when available (exposes a canonical, pullable image ID for containers).
```
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://`)
Related to #32159
c23346f391