feat: add arm64 support

[AppleThomas]

No description provided.

[cursor]

PR Summary

Low Risk
Low risk: confined to CI build tooling, but it changes produced image platforms and may expose cross-arch build issues in Dockerfiles or dependencies.

Overview
Updates the composite GitHub Action ./.github/actions/docker-publish to build and push multi-architecture Docker images by default.

Adds a configurable platforms input (defaulting to linux/amd64,linux/arm64), sets up QEMU for cross-building, and passes the requested platforms through to docker/build-push-action.

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