feat: opt-in multi-arch builds in component-build [ENERGY-2009]

[ckattmann]

What

Adds an opt-in platforms input to component-build.yml (and forwards it through deploy-generic.yml), defaulting to linux/arm64 — i.e. zero behaviour change for every existing caller.

A service that sets platforms: "linux/amd64,linux/arm64" gets a multi-arch manifest pushed instead of a single-arch arm64 image. A setup-qemu-action step (pinned, gated on platforms != 'linux/arm64') handles the cross-arch build on the existing arm64 runner — no new runner pools. Wired in both the blacksmith and self-hosted build paths.

Why

After the ARM64-only switch (#313), component-build hardcodes platforms: linux/arm64 and the architecture input is a no-op, so a service cannot produce an amd64 image. smart-charge runs a single-threaded MILP solver that is ~2× slower on Graviton than the x86 m7i pool it used to run on; we want the ability to fall back to x86. With a multi-arch image, the arm↔x86 node-pool choice becomes a pure, instantly-reversible kube-manifests toggle — and the "exec format error" class of incident can't recur.

Impact / safety

• Default unchanged: existing callers don't pass platforms, so they still do a single native arm64 build. The QEMU step is skipped for them.
• Opt-in cross-arch builds are slower (QEMU emulation) but smart-charge's image is pure-Python (manylinux wheels, no native compilation), so the emulated RUN steps are light.

Follow-up

Consumer change in data-smart-charge deploy.yaml (set platforms) lands after this merges, since passing an undefined input to the reusable workflow errors.

🤖 Generated with Claude Code

https://claude.ai/code/session_01EjBGHdgna5BVQVgwFR2inj

[Copilot]

Pull request overview

This PR adds an opt-in platforms input to the reusable Docker build workflow (component-build.yml) and forwards it through deploy-generic.yml, enabling multi-arch image builds (and manifest list pushes) when explicitly requested while keeping the default behavior as arm64-only.

Changes:

• Introduces a platforms workflow input (default linux/arm64) to control single-arch vs multi-arch Docker builds.
• Adds a pinned, conditional QEMU setup step to support cross-arch builds when platforms is not the default arm64-only value.
• Forwards platforms from deploy-generic.yml into component-build.yml.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
.github/workflows/deploy-generic.yml	Adds a platforms input (default arm64-only) and forwards it to component-build for opt-in multi-arch builds.
.github/workflows/component-build.yml	Adds platforms input, conditionally sets up QEMU for cross-arch builds, and wires platforms into both Blacksmith and self-hosted build actions.

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[ckattmann]

Superseded by monta-app/data-smart-charge#190 — smart-charge will build its own multi-arch image in-repo instead of opting into a shared multi-arch capability. Closing to keep the shared workflow arm64-only. Can revive if another service needs the shared opt-in.