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)

Recreates #86, which was closed unmerged without a stated reason. Rebased cleanly onto current master.

Test plan

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

Part of DEVOPS-1577 / Move to graviton.

[cursor]

PR Summary

Low Risk
CI-only change to how the faucet image is built and published; no application runtime or security logic changes, though image delivery now depends on the shared workflow at @master.

Overview
Replaces the inline build-and-publish-image job (checkout, metadata, Buildx, ghcr.io login, and docker/build-push-action) with the reusable FuelLabs/github-actions docker-build-push.yml@master workflow.

The job keeps the same when conditions and upstream needs, and adds permissions for packages: write and id-token: write. Builds now use build-backend: native for linux/amd64 and linux/arm64 on ubuntu-latest and ubuntu-24.04-arm, with auth-mode: registry-login to ghcr.io and the same image (ghcr.io/fuellabs/faucet) and tag rules (branch, sha, semver).

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