The first build in the container hung for 18+ minutes: the image had only bun's node shim, and
`node --check` under that shim runs each script instead of parsing it. The gates also already
passed before a commit reaches main, so the container now runs clean, token, bundles, tree and
check:sri only. The public exclusions and the refuse-the-swap check still run on every build.
runtime-config.js was written with python3, which the image does not have; bun writes it now.
The build's PID goes to /tmp/build-cdn.pid so it can be stopped without touching the container.