docs: add Node.js deploy guide and runnable example for Datum compute

docs/guides/deploy-a-node-app.md

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+# Deploy a Node.js Web Service on Datum Compute
+
+> Last verified: 2026-06-02 against the `hello-node` example and the live `kraft` / `datumctl compute` CLIs.
+> The complete, ready-to-deploy example for this guide lives in [`examples/hello-node/`](../../examples/hello-node/).
+
+This guide walks you through taking a Node.js HTTP service from source code to a live, reachable instance on Datum compute. By the end you will have:
+
+- A Node.js application packaged as a Unikraft unikernel image
+- The image published to the Unikraft Cloud metro registry
+- A running workload deployed with `datumctl compute deploy`
+- A verified HTTP response from your instance
+
+**What you need before starting:**
+
+- `kraft` (KraftKit) installed and authenticated to your Unikraft Cloud metro. The metro URL and token are supplied to `kraft cloud` commands; this guide assumes they are available as `$UKC_METRO` and `$UKC_TOKEN` in your shell.
+- `datumctl` installed with the compute plugin, authenticated to your Datum Cloud project.
+- Docker (with BuildKit) running locally.
+- Node.js (for local development only — the build happens inside Docker).
+
+---
+
+## 1. Write the application
+
+Create a project directory and add two files.
+
+**`app.js`**
+
+```js
+const http = require('http');
+
+const port = parseInt(process.env.PORT, 10) || 8080;
+
+const server = http.createServer((req, res) => {
+  if (req.url === '/healthz') {
+    res.writeHead(200, { 'Content-Type': 'text/plain' });
+    res.end('ok\n');
+    return;
+  }
+  res.writeHead(200, { 'Content-Type': 'text/plain' });
+  res.end('Hello from Datum (Node)\n');
+});
+
+server.listen(port, () => {
+  console.log('listening on :' + port);
+});
+```
+
+**`package.json`**
+
+```json
+{
+  "name": "hello-node",
+  "version": "1.0.0",
+  "private": true,
+  "main": "app.js",
+  "scripts": {
+    "start": "node app.js"
+  }
+}
+```
+
+The service listens on `$PORT` (default `8080`), answers `/healthz` with `ok`, and has no external dependencies.
+
+---
+
+## 2. Build and publish the unikernel image with `kraft`
+
+### Why Node runs on the `base-compat` runtime
+
+Datum's Unikraft runtime uses an app-elfloader that loads your application as the unikernel entrypoint. Compiled languages (Go, Rust) ship a single fully static binary on the `base:latest` runtime. Node is different: the `node` interpreter is a **dynamically linked** executable — it needs its loader (`ld-musl`) and a few shared libraries present at boot.
+
+For that, Node uses the **`base-compat:latest`** runtime (the binary-compatibility variant of the elfloader) and the rootfs ships the `node` interpreter together with the shared libraries it links. With the loader and libraries present, the dynamic executable boots.
+
+A plain `docker build` OCI image will NOT boot on the runtime. The image must be in the Unikraft Cloud format produced by `kraft`. The `Kraftfile` and `kraft cloud deploy` command handle this packaging.
+
+### Write the Dockerfile
+
+The build installs your dependencies in a regular `node` image, then assembles a minimal `FROM scratch` rootfs containing the interpreter, your app, and exactly the shared libraries `node` needs:
+
+```dockerfile
+FROM node:22-alpine AS build
+WORKDIR /usr/src
+COPY package*.json ./
+RUN npm install
+# npm install with zero deps creates no node_modules dir; ensure it exists so the
+# COPY below succeeds and adding deps later needs no Dockerfile change.
+RUN mkdir -p node_modules
+COPY app.js ./
+# Record node's dynamic-library requirements in the build log for auditing.
+RUN echo "=== ldd node ===" && ldd /usr/local/bin/node || true
+
+FROM scratch
+# The node interpreter.
+COPY --from=build /usr/local/bin/node /usr/bin/node
+# musl dynamic loader + libc (ld-musl is both loader and libc on alpine).
+COPY --from=build /lib/ld-musl-x86_64.so.1 /lib/ld-musl-x86_64.so.1
+# C++/GCC runtime libraries node links against.
+COPY --from=build /usr/lib/libgcc_s.so.1   /usr/lib/libgcc_s.so.1
+COPY --from=build /usr/lib/libstdc++.so.6  /usr/lib/libstdc++.so.6
+COPY --from=build /etc/os-release          /etc/os-release
+# Application + dependency tree.
+COPY --from=build /usr/src/node_modules /usr/src/node_modules
+COPY --from=build /usr/src/app.js       /usr/src/server.js
+```
+
+> **Note:** the scratch image has no package manager, so every shared library `node` links must be copied explicitly — a missing `.so` makes the instance fail to boot. The `ldd /usr/local/bin/node` line in the build log shows exactly which libraries are needed; for stock `node:22-alpine` the three above are the full set.
+
+### Write the Kraftfile
+
+```yaml
+spec: v0.7
+
+name: hello-node
+
+runtime: base-compat:latest
+
+rootfs:
+  source: ./Dockerfile
+  format: erofs
+
+cmd: ["/usr/bin/node", "/usr/src/server.js"]
+```
+
+`runtime: base-compat:latest` is the binary-compatibility elfloader runtime that loads the dynamic `node` executable. `rootfs.source: ./Dockerfile` tells `kraft` to build the rootfs from your Dockerfile.
+
+### Start a BuildKit daemon
+
+`kraft` uses BuildKit to build the rootfs. Start one if you don't already have one running:
+
+```sh
+docker run -d --name buildkit --privileged moby/buildkit:latest
+```
+
+### Build and publish with `kraft cloud deploy --no-start`
+
+Use `kraft` only to build and publish the image — you deploy the running workload with `datumctl compute` in the next step. The `--no-start` (`-S`) flag builds the unikernel package and pushes it to the metro registry **without** starting an instance. It pushes to `index.unikraft.io/datum/<name>`. The `-M` flag sets the memory allocation in MiB and is required.
+
+```sh
+export KRAFTKIT_NO_CHECK_UPDATES=true
+
+kraft cloud --metro "$UKC_METRO" --token "$UKC_TOKEN" \
+  --buildkit-host docker-container://buildkit \
+  deploy --no-start -M 512 --name hello-node \
+  --runtime base-compat:latest --rootfs ./Dockerfile .
+```
+
+After this command completes, your image is available at `index.unikraft.io/datum/hello-node:latest`, ready for Datum compute to deploy.
+
+---
+
+## 3. Deploy on Datum compute
+
+You have two options: a manifest file (recommended for repeatability) or flags.
+
+### Option A — manifest file (recommended)
+
+Create `workload.yaml`:
+
+```yaml
+apiVersion: compute.datumapis.com/v1alpha
+kind: Workload
+metadata:
+  name: hello-node
+  labels:
+    app: hello-node
+spec:
+  template:
+    metadata:
+      labels:
+        app: hello-node
+    spec:
+      runtime:
+        resources:
+          instanceType: datumcloud/d1-standard-2
+        sandbox:
+          containers:
+            - name: app
+              image: index.unikraft.io/datum/hello-node:latest
+              ports:
+                - name: http
+                  port: 8080
+                  protocol: TCP
+      networkInterfaces:
+        - network:
+            name: default
+  placements:
+    - name: default
+      cityCodes:
+        - DFW
+      scaleSettings:
+        minReplicas: 1
+        instanceManagementPolicy: OrderedReady
+```
+
+Deploy it:
+
+```sh
+datumctl compute deploy -f workload.yaml -y
+```
+
+### Option B — flags
+
+```sh
+datumctl compute deploy hello-node \
+  --image=index.unikraft.io/datum/hello-node:latest \
+  --city=DFW \
+  --port=8080 \
+  --min=1
+```
+
+Both forms create (or update) the workload. The `--city` flag accepts one or more city codes; `DFW` targets the US Central region.
+
+---
+
+## 4. Verify the instance is running
+
+List instances and watch for the status to reach `Running`:
+
+```sh
+datumctl compute instances --workload=hello-node
+```
+
+A healthy instance shows `Ready: true` and `Running`. The `EXTERNAL IP` column is populated once the instance is live.
+
+For a detailed view of a single instance, including conditions and any failure reason:
+
+```sh
+datumctl compute instances describe <instance-name>
+```
+
+Once the instance is `Running`, curl the external endpoint. UKC fronts the service with TLS on port 443 and redirects plain HTTP on port 80:
+
+```sh
+# Get the external IP or hostname from the instance list, then:
+curl https://<EXTERNAL-IP>/
+# -> Hello from Datum (Node)
+
+curl https://<EXTERNAL-IP>/healthz
+# -> ok
+```
+
+Use `-k` if the TLS certificate is self-signed in your metro:
+
+```sh
+curl -k https://<EXTERNAL-IP>/
+```
+
+---
+
+## 5. Update the workload
+
+To deploy a new version, rebuild and publish the image (repeating step 2), then redeploy. Using the manifest:
+
+```sh
+kraft cloud --metro "$UKC_METRO" --token "$UKC_TOKEN" \
+  --buildkit-host docker-container://buildkit \
+  deploy --no-start -M 512 --name hello-node \
+  --runtime base-compat:latest --rootfs ./Dockerfile .
+
+datumctl compute deploy -f workload.yaml -y
+```
+
+Or with flags:
+
+```sh
+datumctl compute deploy hello-node \
+  --image=index.unikraft.io/datum/hello-node:latest \
+  --city=DFW \
+  --port=8080
+```
+
+Watch the rollout progress:
+
+```sh
+datumctl compute rollout hello-node
+```
+
+---
+
+## 6. Clean up
+
+```sh
+# Delete the workload and all its instances.
+datumctl compute destroy hello-node -y
+
+# Stop the local BuildKit daemon.
+docker rm -f buildkit
+```
+
+---
+
+## Troubleshooting
+
+### The image fails to boot: missing shared library
+
+If the unikernel console shows a library-not-found error at boot, the rootfs is missing a shared library that `node` (or one of your dependencies) needs. The scratch image has no package manager, so every `.so` must be copied in explicitly. Check:
+
+- The `ldd /usr/local/bin/node` output in the build log lists the libraries `node` itself needs — confirm each is copied into the scratch stage.
+- If you added a **native (node-gyp) addon**, it compiles to additional `.so` files with their own library dependencies. Run `ldd` over the addon's `.node`/`.so` files and copy any libraries they pull in. Pure-JS dependencies need nothing extra.
+- The image was built with `kraft cloud deploy`, not plain `docker build`.
+
+### Instance shows `Ready` but the endpoint doesn't respond
+
+If an instance reports `Ready` but a `curl` to its endpoint hangs or fails, the unikernel may not have booted cleanly. The unikernel console is the source of truth — read it directly:
+
+```sh
+kraft cloud --metro "$UKC_METRO" --token "$UKC_TOKEN" \
+  instance logs <ukc-instance-name>
+```
+
+A healthy boot prints your `listening on :8080` line. A library-not-found error means a `.so` is missing from the rootfs (see above). The `<ukc-instance-name>` appears in the instance's details from `datumctl compute instances describe <instance-name>`.
+
+### Image pull failures on the instance
+
+`datumctl compute instances describe <instance-name>` reports a condition with reason `ImageUnavailable` when the platform cannot pull the image. Confirm:
+
+- The image was pushed to `index.unikraft.io/datum/<name>` (the metro registry), not to an external container registry like GHCR or Docker Hub. The platform pulls from the UKC metro registry.
+- The `kraft cloud deploy` command completed without errors and printed the image reference.
+- The image name in `workload.yaml` matches exactly what `kraft cloud deploy` reported, including the `latest` tag.
+
+### Instance is stuck and not progressing
+
+```sh
+datumctl compute instances describe <instance-name>
+```
+
+Look at the conditions in the output. Common states:
+
+- `QuotaGranted: False` — compute quota has not been provisioned for the project. Contact your platform operator.
+- `Programmed: False` — the instance has not been scheduled to a node yet. This is normal for a few seconds after deploy; if it persists, check that the city code in your workload matches an available location.
+- `Ready: False, reason: SchedulingGatesPresent` — a scheduling prerequisite (such as a network) has not been satisfied. Confirm your project has a `default` Network resource provisioned.

examples/hello-node/.gitignore

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+# Build artifacts produced locally; not committed.
+/node_modules/
+.unikraft/

examples/hello-node/Dockerfile

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+# Multi-stage build for the Node.js runtime proof.
+#
+# Unlike the Go/Rust proofs, Node is NOT a static-PIE binary -- it is a dynamic
+# musl ELF that needs the musl loader and a handful of shared libraries at boot.
+# So this rootfs ships the `node` interpreter AND every shared library it links
+# against, and the unikernel runs on the base-compat:latest runtime (the
+# binary-compatibility / dynamic-loader elfloader variant), NOT base:latest.
+#
+# Stage 1 (node:22-alpine) installs deps and prints `ldd node` so the build log
+# records exactly which shared objects the rootfs must carry. The scratch image
+# below has no package manager, so EVERY library node links must be copied
+# explicitly; a missing .so fails the instance at boot with library-not-found.
+FROM node:22-alpine AS build
+WORKDIR /usr/src
+COPY package*.json ./
+RUN npm install
+# npm install with zero deps creates no node_modules dir; ensure it exists so the
+# COPY below succeeds and adding deps later "just works" with no Dockerfile change.
+RUN mkdir -p node_modules
+COPY app.js ./
+# Record node's dynamic-library requirements in the build log for auditing.
+RUN echo "=== ldd /usr/local/bin/node ===" && ldd /usr/local/bin/node || true
+
+FROM scratch
+# The node interpreter.
+COPY --from=build /usr/local/bin/node /usr/bin/node
+# musl dynamic loader + libc (ld-musl is both loader and libc on alpine).
+COPY --from=build /lib/ld-musl-x86_64.so.1 /lib/ld-musl-x86_64.so.1
+# C++/GCC runtime libraries node links against.
+COPY --from=build /usr/lib/libgcc_s.so.1   /usr/lib/libgcc_s.so.1
+COPY --from=build /usr/lib/libstdc++.so.6  /usr/lib/libstdc++.so.6
+# os-release lets node/libc identify the platform cleanly.
+COPY --from=build /etc/os-release          /etc/os-release
+# Application + (empty for the hello case) dependency tree.
+COPY --from=build /usr/src/node_modules /usr/src/node_modules
+COPY --from=build /usr/src/app.js       /usr/src/server.js

examples/hello-node/Kraftfile

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+# Kraftfile for the Node.js runtime proof.
+#
+# Node is a dynamic musl ELF, so it runs on base-compat:latest (the
+# binary-compatibility / dynamic-loader elfloader variant), NOT base:latest used
+# by the static-PIE Go/Rust proofs. The rootfs (built from the multi-stage
+# Dockerfile) carries the node interpreter plus every shared library it links.
+#
+# Build/push (push-only, do not start):
+#   kraft cloud --metro <metro>/v1 --token <token> \
+#     --buildkit-host docker-container://buildkit \
+#     deploy --no-start -M 512 --name hello-node \
+#     --runtime base-compat:latest --rootfs ./Dockerfile .
+spec: v0.7
+
+name: hello-node
+
+runtime: base-compat:latest
+
+rootfs:
+  source: ./Dockerfile
+  format: erofs
+
+cmd: ["/usr/bin/node", "/usr/src/server.js"]