ci: use Docker Buildx + ECR registry cache for integration-test prepare-cluster

[amir-deris]

Summary

• Replace uncached docker build in prepare-cluster with Docker Buildx + ECR registry-backed layer cache, using dedicated cache tags (build-cache:integration-localnode, build-cache:integration-rpcnode) separate from the release build-cache:shared.
• Add build-seid-in-localnode-ci Makefile target that compiles seid inside an already-loaded localnode container, avoiding a redundant image rebuild.
• Fork PRs degrade gracefully to a cold build: AWS OIDC login uses continue-on-error, and cache refs are conditionally empty when auth fails. Cache writes (cache-to) are further restricted to push events to avoid cache thrashing on PRs.

Expected impact

Step	Cold build	Warm (Dockerfiles unchanged)
localnode image	~10–20+ min	~1–3 min
rpcnode image	~2–5 min	~30s–1 min
seid compile	unchanged	unchanged

[cursor]

PR Summary

Low Risk
Changes are limited to CI workflow and Makefile helpers; runtime chain behavior is unchanged, with graceful fallback when ECR is unavailable.

Overview
The prepare-cluster job in integration-test.yml no longer builds images via make build-integration-ci-artifacts. It now uses Docker Buildx to build and load sei-chain/localnode and sei-chain/rpcnode, with registry cache in Amazon ECR when AWS login succeeds.

AWS OIDC (id-token: write) and optional configure-aws-credentials / ECR login steps were added; both use continue-on-error so fork PRs without credentials still get cold builds. Cache writes (cache-to) run only on push events to limit PR cache churn. Checkout was bumped to v5, and seid is built with the new make build-seid-in-localnode-ci target, which assumes images are already present and only runs the in-container compile (via ensure-integration-ci-images).

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[github-actions]

The latest Buf updates on your PR. Results from workflow Buf / buf (pull_request).

Build	Format	Lint	Breaking	Updated (UTC)
✅ passed	✅ passed	✅ passed	✅ passed	Jun 3, 2026, 12:49 PM

[cursor]

Cursor Bugbot has reviewed your changes using default effort and found 1 potential issue.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 58.28%. Comparing base (2151ba0) to head (d4b1c30).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3539      +/-   ##
==========================================
- Coverage   59.12%   58.28%   -0.85%     
==========================================
  Files        2213     2140      -73     
  Lines      182814   174431    -8383     
==========================================
- Hits       108096   101660    -6436     
+ Misses      64993    63725    -1268     
+ Partials     9725     9046     -679     

Flag	Coverage Δ
sei-db	70.41% <ø> (ø)
sei-db-state-db	?

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