docs: add serverless Node.js example mounting a function via ConfigMap

[scotwells]

What this delivers

A runnable example showing a serverless pattern on Datum compute: a generic Node.js runtime image with no application code baked in, where the function (index.js) is supplied at deploy time via a ConfigMap mounted at /app, and Node executes the mounted file. One image serves any function, and swapping the code is a ConfigMap edit + restart — no image rebuild or repush. It complements examples/hello-node/ (#132), which bakes the app into the image; this example pulls the app back out and delivers it as data.

How it works

The generic image is built once on the base-compat:latest runtime with an erofs rootfs. The Kraftfile's cmd points node at /app/index.js — a mounted path, not a baked one. The Workload mounts the js-function ConfigMap at /app, so the function arrives as a read-only rom in the microVM at boot. Editing the function and restarting recreates the instance against the new content.

The js-function ConfigMap is generated from index.js by a Kustomize configMapGenerator, so index.js is the single source of truth — there is no second, hand-maintained copy of the function to drift. disableNameSuffixHash: true keeps the name stable, since the Workload references the ConfigMap by name and is deployed out-of-band via datumctl (a content-hash suffix would break the mount and buys nothing here).

What's included

examples/serverless-js-configmap/, ready to deploy as-is:

• Dockerfile — multi-stage build of the generic runtime: a scratch image carrying only the node interpreter and its musl/libgcc/libstdc++ shared libraries. No JS.
• Kraftfile — spec: v0.7, base-compat:latest, rootfs.format: erofs, cmd: ["/usr/bin/node", "/app/index.js"].
• index.js — the function and single source of truth for the ConfigMap: a dependency-free Node stdlib HTTP service on $PORT (default 8080), with a versioned response and startup log so a code swap is observable.
• kustomization.yaml — a configMapGenerator that builds the js-function ConfigMap from index.js (kubectl apply -k .).
• workload.yaml — the Datum Workload mounting the ConfigMap at /app (port 8080, d1-standard-2, DFW, minReplicas: 1).
• README.md — build + push, apply-ConfigMap (-k), deploy, validate, and the code-swap steps, plus the runtime/erofs/empty-env pitfalls.

Validation

Verified end to end on the ash-sore-hamster metro: the ConfigMap mounts as a read-only rom at /app (mount /dev/ukp_rom1 -> /app in the boot log), Node executes the mounted file (serverless-js: v1 listening on :8080), and editing only the function swapped the live response v1 -> v2 on the same image SHA — no rebuild. (The generator produces a byte-identical js-function ConfigMap, so this validation is unchanged by the Kustomize refactor.)

Dependency

The referenced ConfigMap/Secret delivery this relies on (base-compat:latest + erofs) is delivered by #129; this example is most useful once that lands.

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