chore(docker): native multi-arch via shared workflow

[gondoi]

Summary

• Migrate container builds to FuelLabs/github-actions/.github/workflows/docker-build-push.yml@master
• Native multi-arch: linux/amd64 + linux/arm64 (ubuntu-latest / ubuntu-24.04-arm)

Test plan

• CI passes on PR (amd64 + arm64 build jobs)
• Image manifest lists both architectures after merge

Part of DEVOPS-1249 / Move to graviton.

Depends on github-actions master with checkout/LFS inputs (DEVOPS-1248).

Made with Cursor

[cursor]

PR Summary

Low Risk
CI-only change to how the Prometheus exporter image is built and published; no application runtime code is modified.

Overview
Replaces the inline build-exporter job (checkout, metadata, Buildx, dual registry login, build-push with registry cache on BuildJet) with a reusable call to FuelLabs/github-actions docker-build-push.yml@master, keeping the same ghcr.io/fuellabs/prometheus-fuel-exporter image and tag/flavor rules.

Adds workflow-level concurrency (cancel in-progress per PR/ref), permissions (contents, packages, id-token), and native multi-arch builds for linux/amd64 and linux/arm64 via build-backend: native and separate AMD64/ARM runners. Removes the previous docker.io login, BuildJet runner, and local registry cache configuration from this repo’s workflow.

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