fix macos local dev

[Sayan-]

life happens fast

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Note

Medium Risk
Changes the build/run platform for the chromium Docker images, which can affect local dev behavior and may break workflows that relied on native arm64 images without explicitly overriding the platform.

Overview
For the chromium-headful and chromium-headless images, the build scripts now pass --platform "$DOCKER_PLATFORM" to docker build, and the run scripts include --platform "$DOCKER_PLATFORM" in docker run.

ensure-common-build-run-vars.sh introduces DOCKER_PLATFORM (defaulting to linux/amd64) with rationale for Chrome-for-Testing on Apple Silicon, allowing overrides for custom arm64 images.

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