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

Medium Risk
Changes release and PR CI paths and depend on an external reusable workflow at master; runtime app logic is unchanged but a broken or mismatched workflow could block merges or publish wrong images.

Overview
CI now delegates container build and push to the shared FuelLabs/github-actions docker-build-push workflow instead of inline Buildx steps. Images are built for linux/amd64 and linux/arm64 (native runners), published to ghcr.io, and the image job also runs on pull requests (not only master / manual dispatch).

PR frontend tests no longer build into a local registry: they depend on build-and-publish-image, log in to GHCR, pull the built image by digest (linux/amd64), and run it for npm build/test.

The Dockerfile build stage base image moves from Rust 1.82 to 1.86 (cargo-chef). The workflow drops the top-level REGISTRY env var in favor of inputs on the reusable workflow.

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