Pin smoke Dockerfile to amd64; fail fast on missing overlay template

[eugenevinitsky]

What

• tests/smoke_tests/Dockerfile: pin FROM --platform=linux/amd64 python:3.11-slim-bookworm and extend the header comment to state that the image is x86-64-only by definition (bit-exact AVX2 golden under qemu-x86_64-static; setup.py ships only the amd64 raylib archive) and that ARM hosts run it under emulation — pass --platform linux/amd64 to docker build/run if the FROM pin is not honored.
• scripts/setup_container.sh: in create_overlay(), add a fail-fast guard before the cp — if $OVERLAY_TEMPLATE does not exist, print a clear error explaining the default path exists only on NYU Greene, with two remediations (export OVERLAY_TEMPLATE, or apptainer overlay create --size 15360 --create-dir /ext3 "$OVERLAY_PATH", available as singularity overlay create on singularity >= 3.8), then exit 1.
• README.md: document that the /share/apps defaults (IMAGE_PATH, OVERLAY_TEMPLATE) are NYU-Greene-only, that OVERLAY_TEMPLATE is just a gzipped empty ext3 image, and how to substitute a site template / apptainer overlay create / any CUDA-capable .sif.
• docs/cluster_training.md: one short comment in the quick-overview block pointing non-NYU users at OVERLAY_TEMPLATE/IMAGE_PATH exports or apptainer overlay create (see README defaults table).
• AGENTIC_PR: French poem per repo convention for agent-created PRs.

Why

The smoke image is x86-64-only by design, but without a platform pin Apple-Silicon Docker builds an arm64 image that fails at link time (raylib 5.5 has no linux_arm64 asset). Separately, create_overlay() blindly cp'd the NYU-Greene-only template under set -e, so everyone off Greene got a bare "No such file or directory" with no hint of what the template is or how to substitute it.

Notes

• Verification: bash -n scripts/setup_container.sh passes. The guard was exercised for real (safe off-cluster: only mkdir -p on a scratchpad-scoped CONTAINER_DIR precedes it): with OVERLAY_TEMPLATE=/nonexistent/... the script prints the full remediation message and exits 1. git diff confirms the Dockerfile's only functional change is the FROM platform pin; docker build was not run (Docker unavailable in this environment).
• Risks: minimal. The pin only makes the existing implicit amd64 requirement explicit; on amd64 hosts behavior is unchanged. The guard only fires in the failure path that previously crashed with a worse message. README/docs changes are prose-only.
• Merge-order: the docs/cluster_training.md hunk was kept to a 2-line comment insertion because another PR adds a section near the top of that file; if that PR lands first, this one may need a trivial rebase (comment placement only).

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🤖 Generated with Claude Code