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
Only GitHub Actions wiring changes; runtime app code is untouched, though image publish behavior depends on the external reusable workflow.

Overview
The build-and-publish-image job no longer runs Docker build steps inline. It now calls the shared reusable workflow FuelLabs/github-actions/.github/workflows/docker-build-push.yml@master, with explicit permissions for packages: write and id-token: write.

Publishing still targets ghcr.io/fuellabs/faucet with the same tag rules (branch ref, SHA, semver on release), but builds use build-backend: native for linux/amd64 and linux/arm64, on ubuntu-latest and ubuntu-24.04-arm respectively, instead of a single-runner Buildx push flow.

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