Automatic merge from submit-queue (batch tested with PRs 43726, 43643)
Make a smaller redis image for testing, based on Alpine.
**What this PR does / why we need it**:
This shrinks gcr.io/google_containers/redis from 400MB to 5MB, which should reduce flakes.
**Which issue this PR fixes**:
fixes#43631
**Release note**:
```release-note
NONE
```