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

internal/proxy/proxy.go

@@ -13,6 +13,7 @@ import (
 	"sneak.berlin/go/smallwebwaf/internal/config"
 	"sneak.berlin/go/smallwebwaf/internal/lookup"
 	"sneak.berlin/go/smallwebwaf/internal/ratelimit"
+	"sneak.berlin/go/smallwebwaf/internal/requestlog"
 )
 
 // The request line and headers a client may send, and how long a
@@ -34,6 +35,10 @@ const (
 	appIdleConnTimeout = 90 * time.Second
 )
 
+// HealthPath is smallwebwaf's health endpoint, which the container's
+// health check asks.
+const HealthPath = "/_smallwebwaf/healthz"
+
 // Params are what New needs.
 type Params struct {
 	Config *config.Config
@@ -111,6 +116,15 @@ func (h *handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	rq := h.newRequest(w, r)
 	defer rq.finish()
 
+	// The health endpoint is answered at once, before any check, so that
+	// a health checker is never refused. It does not ask the app.
+	if r.Method == http.MethodGet && r.URL.Path == HealthPath {
+		rq.line.Action = requestlog.ActionAdmin
+		_, _ = io.WriteString(rq.out, "ok\n")
+
+		return
+	}
+
 	refused := rq.check(r.Context())
 	if refused != nil {
 		rq.answer(*refused)