Fix Docker x64 runner architecture mismatch on ARM hosts

[depoll]

Summary

• enforce Docker image architecture checks after pull and after architecture-specific retagging
• force Docker container creation to use the requested platform (linux/amd64 or linux/arm64)
• add backend unit tests for architecture normalization and mismatch failure behavior

Validation

• npm run typecheck -w backend
• npm run test -w backend -- tests/dockerRunner.test.ts
• PORT=40123 npm run test -w backend

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Note

Medium Risk
Changes affect runner provisioning and Docker image selection; failures will now surface earlier as hard errors if images/platforms don’t match, which could impact deployments with custom images/tags.

Overview
Fixes Docker-based GitHub Actions runners on mixed-arch hosts by normalizing and enforcing image architecture during pullRunnerImage, failing fast if the pulled or retagged image doesn’t match the requested amd64/arm64.

Forces container creation to use the requested linux/<arch> platform (via createContainer({ platform })) to avoid accidental host-arch execution. Adds Vitest coverage for architecture normalization and mismatch errors.

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[Copilot]

Pull request overview

This PR hardens Docker runner provisioning on ARM hosts by explicitly enforcing image architecture correctness and ensuring containers are created using the requested Docker platform, preventing x64/arm64 mismatch failures.

Changes:

• Add architecture alias normalization and a reusable architecture mismatch assertion for Docker images.
• Verify image architecture both after pulling the base image and after retagging to an architecture-specific tag.
• Force dockerode.createContainer to target the requested linux/amd64 or linux/arm64 platform, and add unit tests for the new helpers.

Reviewed changes

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

File	Description
backend/src/services/dockerRunner.ts	Introduces architecture normalization/assertion helpers, enforces image-arch checks after pull/tag, and sets container creation platform.
backend/tests/dockerRunner.test.ts	Adds Vitest coverage for architecture alias normalization and mismatch error behavior.

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