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

share/smallwebwaf.run

@@ -0,0 +1,12 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+# runit's run script for smallwebwaf, run again whenever smallwebwaf
+# exits; the wait spaces out the restarts. exec, so that the signal
+# `sv stop` sends reaches smallwebwaf itself.
+main() {
+    sleep 1
+    exec chpst -u smallwebwaf:smallwebwaf /usr/local/bin/smallwebwaf
+}
+
+main "$@"