fix: resolve disk-space exhaustion in backward-compat CI

[Copilot]

The backward-compat-test CI job was failing with no space left on device when loading Docker images to the kind cluster. The backward-compat workflow consumes more disk than the regular e2e test because it first pulls old Fluid images via helm install from the chart repo before building and loading 10 new images—leaving ~36 MB free, not enough for docker save temp files.

Changes

• .github/workflows/backward-compatibility-e2e.yml: Add a "Free disk space" step before the Docker build that removes unused pre-installed runner tools (dotnet ~1.5 GB, Android SDK ~6 GB, GHC, CodeQL) to reclaim headroom before any image work begins.

• .github/scripts/build-all-images.sh: Remove each image from local Docker storage immediately after loading it into kind. Once an image is in the kind node, the local copy is not needed—keeping it only contends for the same disk space required by docker save for the next image.

for img in "${images[@]}"; do
    echo "Loading image ${img} to kind cluster..."
    kind load docker-image "${img}" --name "${KIND_CLUSTER}"
    echo "Removing local image ${img} to free disk space..."
    docker rmi "${img}" || echo "Warning: Failed to remove image ${img}, continuing..."
done

[fluid-e2e-bot]

Hi @Copilot. Thanks for your PR.

I'm waiting for a fluid-cloudnative member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 63.56%. Comparing base (da69206) to head (2758112).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6002   +/-   ##
=======================================
  Coverage   63.56%   63.56%           
=======================================
  Files         479      479           
  Lines       33276    33276           
=======================================
  Hits        21151    21151           
  Misses      10445    10445           
  Partials     1680     1680           

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[cheyang]

Looks good. This is a standard and well-tested pattern for reclaiming disk space on GitHub Actions runners.

The two-pronged approach works well:

1. Pre-build runner cleanup removes ~14GB of pre-installed toolchains (.NET, Android SDK, GHC, CodeQL) that this workflow never uses.
2. Eager per-image removal in build-all-images.sh frees each Docker image right after loading it into kind, preventing accumulation during the loop.

Verified:

• Only CI files touched (.github/scripts/ and .github/workflows/)
• No secret exfiltration or unexpected network calls
• docker image prune --all --force is placed after kind cluster creation, so the kindest/node image is protected by its running container
• Error handling (|| echo "Warning...") is appropriate and won't break the pipeline

LGTM — this should unblock the backward-compat CI jobs that have been running out of disk.

[fluid-e2e-bot]

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