Fix unauth pulls from dockerhub (#4272)

# Description of Changes

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here. -->

Apparently we ran too many unity testsuite jobs today and we got this:
```
Unable to find image 'unityci/editor:ubuntu-2022.3.32f1-linux-il2cpp-3' locally
ubuntu-2022.3.32f1-linux-il2cpp-3: Pulling from unityci/editor
docker: error from registry: You have reached your unauthenticated pull rate limit. https://www.docker.com/increase-rate-limit
```

So now we login before pulling to increase our limit.

# API and ABI breaking changes

None

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corresponding GitHub label. -->

# Expected complexity level and risk

1 - this just logs us into docker before running the test

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# Testing

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reviewers to do,
so that you're confident that all the changes work as expected! -->

- [x] CI still passes

Author: jdetter

.github/workflows/ci.yml

@@ -841,6 +841,12 @@ jobs:
           key: Unity-${{ github.head_ref }}
           restore-keys: Unity-
 
+      - name: Login to DockerHub
+        uses: docker/login-action@v2
+        with:
+          username: ${{ vars.DOCKERHUB_USERNAME }}
+          password: ${{ secrets.DOCKERHUB_PASSWORD }}
+
       - name: Run Unity tests
         uses: game-ci/unity-test-runner@v4
         with: