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

script/bootstrap

@@ -131,6 +131,7 @@ main() {
 
     if missing make; then pkg_install gnumake make make make; fi
     if missing git; then pkg_install git git git git; fi
+    if missing curl; then pkg_install curl curl curl curl; fi
     if missing gofmt; then pkg_install go golang go go; fi
 
     ensure_node