ci: add Docker build cache

[aishwaryabandapelly-ai]

Summary

Adds GitHub Actions cache support to the Docker build steps in publish-docker.yml.

This updates the existing docker/build-push-action steps to use BuildKit's GitHub Actions cache backend with platform-specific cache scopes.

Changes

• Adds a cache-scope value for each Docker build matrix platform
• Adds cache-from: type=gha to Docker build steps
• Adds cache-to: type=gha,mode=max to Docker build steps
• Keeps cache scopes separate for linux/amd64 and linux/arm64

Why

This addresses #791 by improving Docker build performance through reusable Docker layer caching.

Testing

This is a GitHub Actions workflow-only change. I validated the diff locally with:

git diff --check

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: aishwaryabandapelly-ai / name: aishwaryabandapelly-ai (e3fb8dd)

[opentelemetry-pr-dashboard]

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Waiting on reviewers · refreshed 2026-08-04 14:26 UTC

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.5%. Comparing base (b5978df) to head (e3fb8dd).
⚠️ Report is 14 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #1647     +/-   ##
=======================================
- Coverage   81.5%   81.5%   -0.1%     
=======================================
  Files        130     130             
  Lines      11616   11616             
=======================================
- Hits        9475    9473      -2     
- Misses      2141    2143      +2     

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