The image apps build FROM, with its health check (closes #45)

The Dockerfile's last stage is now the image of "Deployment" in SPEC.md:
Ubuntu 26.04 with ca-certificates, nix-bin and runit from a dated
snapshot whose InRelease files are checked by hash, nixpkgs from its
release file checked by SHA-256, runsvinit built at a fixed commit, and
smallwebwaf as a runit service. smallwebwaf answers
/_smallwebwaf/healthz, and `smallwebwaf healthcheck`, which takes no
further argument, is the image's HEALTHCHECK. script/example-app builds
an app on the image and checks it end to end.

The Nix profile comes last on the PATH: first, busybox from nixpkgs
replaced runit's own runsvdir and sv. SPEC.md is corrected to match
what was built.

Model: opus-5-5

deploy/example-app/Dockerfile

@@ -0,0 +1,20 @@
+# An app built on the smallwebwaf image, as under "Deployment" in
+# SPEC.md, which script/example-app builds and checks. The app is
+# busybox's web server, from the nixpkgs in the image, serving one page;
+# a real app copies in its own binary instead.
+#
+# A real app names the smallwebwaf image by digest. This one takes the
+# image script/example-app has just built, or else the one `make docker`
+# builds.
+ARG SMALLWEBWAF_IMAGE=smallwebwaf
+FROM ${SMALLWEBWAF_IMAGE}
+
+# Packages the app needs, from the nixpkgs in the image.
+RUN nix-env -iA nixpkgs.busybox
+
+# The app's page, and a user of its own to run it.
+RUN mkdir /var/www && echo 'hello from the example app' > /var/www/index.html
+RUN useradd --system --no-create-home --shell /usr/sbin/nologin app
+
+# The app's runit service.
+COPY --chmod=755 app.run /etc/service/app/run

deploy/example-app/app.run

@@ -0,0 +1,9 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+main() {
+    sleep 1
+    exec chpst -u app:app busybox httpd -f -p 127.0.0.1:8081 -h /var/www
+}
+
+main "$@"