Automatic merge from submit-queue
rkt: Convert image name to be a valid acidentifier
**Release note**:
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```release-note
Fix a bug under the rkt runtime whereby image-registries with ports would not be fetched from
```
This fixes a bug whereby an image reference that included a port was not
recognized after being downloaded, and so could not be run
This is the quick-and-simple fix. In the longer term, we'll want to refactor image logic a bit more to handle the many special cases that the current code does not, mostly related to library images on dockerhub.
/cc @yifan-gu @kubernetes/sig-rktnetes
c04aec3462