build: run all linting in Docker via Dockerfile.lint (closes #104)

golangci-lint now runs only inside a container, never on the host.
script/lint builds a hash-pinned root Dockerfile.lint; the nix-shell and
host golangci-lint paths are gone. A per-run CACHEBUST build-arg is
folded into the lint step's cache key, so the linter re-executes on every
run and an unchanged tree cannot return a cached success having linted
nothing; script/lint fails a build that did not run the linter.

Dockerfile's lint stage runs golangci-lint directly, since make lint now
builds a container and there is no Docker inside a build. It is the same
image and config. golangci-lint config verify is left out: it fetches its
schema over an unpinned live HTTPS call, which REPO_POLICIES.md forbids.

Model: opus-4-8

Dockerfile

@@ -13,9 +13,12 @@ RUN go mod download
 # Copy source code
 COPY . .
 
-# Run formatting check and linter
+# Run formatting check and linter. The linter is invoked directly, not
+# via `make lint`: `make lint` now builds Dockerfile.lint, and there is
+# no Docker inside a Docker build. This is the same linter, image, and
+# config that Dockerfile.lint and script/lint run.
 RUN make fmt-check
-RUN make lint
+RUN golangci-lint run --config .golangci.yml ./...
 
 # Build stage
 # golang:1.25.4-alpine, 2026-02-25

Dockerfile.lint

@@ -0,0 +1,41 @@
+# Dockerfile.lint: the one and only path that runs golangci-lint.
+#
+# golangci-lint is never installed on the host; it runs only inside this
+# build. A clean build of this file therefore IS a clean lint over the
+# whole tree. It runs the same linter and config as Dockerfile's lint
+# stage, pinned to the same image so the two cannot drift to different
+# linter versions.
+#
+# golangci/golangci-lint:v2.12.2-alpine, 2026-08-07
+FROM golangci/golangci-lint:v2.12.2-alpine@sha256:91b27804074a0bacea298707f016911e60cf0cdbc6c7bf5ccacb5f0606d18d60
+
+# pixa is CGO/libvips: the type-aware linters compile every package, so
+# this image needs the same C libraries the build does.
+RUN apk add --no-cache build-base vips-dev libheif-dev pkgconfig
+
+WORKDIR /src
+
+# Modules first for layer caching; go.mod/go.sum settle this layer's
+# result, so it may safely be reused between runs.
+COPY go.mod go.sum ./
+RUN go mod download
+
+COPY . .
+
+# Caching is deliberately waived for the lint step: an unchanged tree
+# must still run the linter, not return a cached success in well under a
+# second having linted nothing. CACHEBUST carries a value that differs
+# on every run (script/lint supplies it and refuses to build without
+# one). The lint RUN below references it, so BuildKit cannot serve that
+# step from cache. Keep the ${CACHEBUST} reference on that step: dropping
+# it lets the linter cache again and report a green that linted nothing.
+ARG CACHEBUST
+RUN test -n "${CACHEBUST}" || { \
+        echo "Dockerfile.lint requires the CACHEBUST build-arg; build it via script/lint." >&2; \
+        exit 1; }
+
+# `golangci-lint config verify` is deliberately not run: it fetches its
+# JSON schema over an unpinned live HTTPS call, which REPO_POLICIES.md
+# forbids for external references.
+RUN echo "pixa-lint: running golangci-lint (${CACHEBUST})" && \
+    golangci-lint run --config .golangci.yml ./...

TODO.md

@@ -29,14 +29,15 @@ P1: implement blocked networks configuration to extend SSRF protection
 
 # Completed Steps
 
-- 2026-09-21 http.Server hardening (closes #92): added
-  `HTTPReadHeaderTimeout` (10s, bounds the slowloris header dribble) and
-  `HTTPIdleTimeout` (120s, bounds keep-alive reuse) alongside the
-  existing timeouts and wired them onto the server; added a `LimitBody`
-  middleware capping the two form POST bodies (`POST /`, `POST /generate`)
-  at `MaxFormBytes` (1 MiB) and returning 413, applied ahead of the CSRF
-  middleware so an oversized body is refused as 413 rather than being read
-  as a missing CSRF token (403); left `WriteTimeout` at 60s unchanged
+- 2026-09-21 run all linting in Docker via `Dockerfile.lint` +
+  `script/lint` (closes #104): `script/lint` builds a hash-pinned root
+  `Dockerfile.lint`, and no host or nix-shell `golangci-lint` path
+  remains; a per-run `CACHEBUST` build-arg forces the lint step to
+  execute every run, so an unchanged tree cannot return a cached green
+  that linted nothing; `Dockerfile`'s lint stage runs `golangci-lint`
+  directly, since `make lint` now builds a container and there is no
+  Docker inside a build; `golangci-lint config verify` stays out, as it
+  fetches its schema over an unpinned live HTTPS call
 - 2026-08-07 update golangci-lint to v2.12.2 with the canonical
   `.golangci.yml` (v2 schema, `default: all` minus six disabled
   linters, `lll` 88, tests included): bumped the pinned

internal/handlers/bodylimit.go

@@ -1,45 +0,0 @@
-package handlers
-
-import (
-	"errors"
-	"net/http"
-)
-
-// MaxFormBytes bounds the request body accepted on the HTML form POST
-// routes (POST / and POST /generate). The forms carry a handful of short
-// fields, so 1 MiB is generous while making the bound explicit rather than
-// resting on ParseForm's incidental 10 MB cap.
-const MaxFormBytes = 1 << 20 // 1 MiB
-
-// LimitBody returns middleware that caps the request body on POST requests
-// at maxBytes and rejects an oversized body with 413 Request Entity Too
-// Large.
-//
-// It parses the form here, before the CSRF middleware reads the token from
-// it. The CSRF middleware reads the token with PostFormValue, which
-// swallows a parse error, so if the body were only capped there an
-// oversized body would read as a missing token and be refused as 403. By
-// parsing under the cap first, an oversized body is refused as 413. A
-// successful parse is cached on the request, so the CSRF check and the
-// handler reuse it rather than reading the body again.
-func (s *Handlers) LimitBody(maxBytes int64) func(http.Handler) http.Handler {
-	return func(next http.Handler) http.Handler {
-		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
-			if r.Method == http.MethodPost {
-				r.Body = http.MaxBytesReader(w, r.Body, maxBytes)
-
-				err := r.ParseForm()
-
-				var tooLarge *http.MaxBytesError
-				if errors.As(err, &tooLarge) {
-					http.Error(w, "Request body too large",
-						http.StatusRequestEntityTooLarge)
-
-					return
-				}
-			}
-
-			next.ServeHTTP(w, r)
-		})
-	}
-}

internal/handlers/bodylimit_internal_test.go

@@ -1,177 +0,0 @@
-package handlers
-
-import (
-	"log/slog"
-	"net/http"
-	"net/url"
-	"strings"
-	"testing"
-
-	"github.com/go-chi/chi/v5"
-
-	"sneak.berlin/go/pixa/internal/config"
-	"sneak.berlin/go/pixa/internal/encurl"
-	"sneak.berlin/go/pixa/internal/session"
-)
-
-// Form field names and a throwaway source image URL for the body-limit
-// tests.
-const (
-	sourceURLField = "url"
-	testSourceURL  = "https://example.com/a.jpg"
-)
-
-// newBodyLimitTestRouter mirrors the production wiring for the form POST
-// routes (see server.SetupRoutes): LimitBody sits in front of the CSRF
-// middleware, which sits in front of the handlers. maxBytes is the body
-// cap under test, so a test can trip the limit with a small body.
-func newBodyLimitTestRouter(
-	t *testing.T, maxBytes int64,
-) (*Handlers, http.Handler) {
-	t.Helper()
-
-	cfg := &config.Config{SigningKey: testSigningKey, Debug: true}
-
-	sessMgr, err := session.NewManager(testSigningKey)
-	if err != nil {
-		t.Fatalf("session.NewManager() error = %v", err)
-	}
-
-	encGen, err := encurl.NewGenerator(testSigningKey)
-	if err != nil {
-		t.Fatalf("encurl.NewGenerator() error = %v", err)
-	}
-
-	protect, err := newCSRFProtect(testSigningKey, cfg.Debug)
-	if err != nil {
-		t.Fatalf("newCSRFProtect() error = %v", err)
-	}
-
-	h := &Handlers{
-		log:         slog.New(slog.DiscardHandler),
-		config:      cfg,
-		sessMgr:     sessMgr,
-		encGen:      encGen,
-		csrfProtect: protect,
-	}
-
-	r := chi.NewRouter()
-	r.Group(func(r chi.Router) {
-		r.Use(h.LimitBody(maxBytes))
-		r.Use(h.CSRF())
-		r.Get("/", h.HandleRoot())
-		r.Post("/", h.HandleRoot())
-		r.Post("/generate", h.HandleGenerateURL())
-	})
-
-	return h, r
-}
-
-// TestOversizedLoginPostRejectedBeforeCSRF is the core regression: an
-// oversized POST / carrying an otherwise valid CSRF cookie and token must
-// be rejected with 413. If the body limit ran after CSRF, the truncated
-// body would read as a missing token and return 403; if it ran after the
-// handler, a valid token would return 303. Getting 413 proves the limit
-// fires before CSRF parses the form.
-func TestOversizedLoginPostRejectedBeforeCSRF(t *testing.T) {
-	t.Parallel()
-
-	_, srv := newBodyLimitTestRouter(t, 16)
-
-	cookies, token := csrfCredentials(t, srv, nil)
-
-	rec := postForm(srv, "/", cookies, url.Values{
-		loginKeyField:  {testSigningKey},
-		csrfTokenField: {token},
-	})
-
-	if rec.Code != http.StatusRequestEntityTooLarge {
-		t.Errorf("oversized POST / status = %d, want %d",
-			rec.Code, http.StatusRequestEntityTooLarge)
-	}
-}
-
-// TestOversizedGeneratePostRejectedBeforeCSRF is the same regression for
-// POST /generate, which also parses a form behind CSRF.
-func TestOversizedGeneratePostRejectedBeforeCSRF(t *testing.T) {
-	t.Parallel()
-
-	h, srv := newBodyLimitTestRouter(t, 16)
-
-	sessionCookie := newSessionCookie(t, h)
-
-	cookies, token := csrfCredentials(t, srv, []*http.Cookie{sessionCookie})
-	cookies = append(cookies, sessionCookie)
-
-	rec := postForm(srv, "/generate", cookies, url.Values{
-		sourceURLField: {testSourceURL},
-		csrfTokenField: {token},
-	})
-
-	if rec.Code != http.StatusRequestEntityTooLarge {
-		t.Errorf("oversized POST /generate status = %d, want %d",
-			rec.Code, http.StatusRequestEntityTooLarge)
-	}
-}
-
-// TestWithinLimitLoginPostSucceeds verifies the limit does not disturb a
-// normal request: under the production cap, a valid login still parses and
-// establishes a session (303). This guards against the body limit
-// consuming or corrupting the form the CSRF check and handler depend on.
-func TestWithinLimitLoginPostSucceeds(t *testing.T) {
-	t.Parallel()
-
-	_, srv := newBodyLimitTestRouter(t, MaxFormBytes)
-
-	cookies, token := csrfCredentials(t, srv, nil)
-
-	rec := postForm(srv, "/", cookies, url.Values{
-		loginKeyField:  {testSigningKey},
-		csrfTokenField: {token},
-	})
-
-	if rec.Code != http.StatusSeeOther {
-		t.Fatalf("within-limit POST / status = %d, want %d",
-			rec.Code, http.StatusSeeOther)
-	}
-
-	var authed bool
-
-	for _, c := range rec.Result().Cookies() {
-		if c.Name == session.CookieName && c.Value != "" {
-			authed = true
-		}
-	}
-
-	if !authed {
-		t.Error("within-limit valid login did not set a session cookie")
-	}
-}
-
-// TestWithinLimitGeneratePostSucceeds is the same non-regression check for
-// POST /generate.
-func TestWithinLimitGeneratePostSucceeds(t *testing.T) {
-	t.Parallel()
-
-	h, srv := newBodyLimitTestRouter(t, MaxFormBytes)
-
-	sessionCookie := newSessionCookie(t, h)
-
-	cookies, token := csrfCredentials(t, srv, []*http.Cookie{sessionCookie})
-	cookies = append(cookies, sessionCookie)
-
-	rec := postForm(srv, "/generate", cookies, url.Values{
-		sourceURLField: {testSourceURL},
-		"format":       {"jpeg"},
-		csrfTokenField: {token},
-	})
-
-	if rec.Code != http.StatusOK {
-		t.Fatalf("within-limit POST /generate status = %d, want %d",
-			rec.Code, http.StatusOK)
-	}
-
-	if !strings.Contains(rec.Body.String(), "/v1/e/") {
-		t.Error("within-limit generate response did not contain a generated URL")
-	}
-}

internal/middleware/middleware.go

@@ -21,33 +21,6 @@ import (
 // CORSMaxAgeSeconds is the max age for CORS preflight cache (24 hours).
 const CORSMaxAgeSeconds = 86400
 
-// HSTSValue is the Strict-Transport-Security header value: one year with
-// includeSubDomains. Emitted unconditionally even though pixa listens plain
-// HTTP behind a TLS-terminating proxy; browsers ignore an HSTS header received
-// over plaintext (RFC 6797 section 8.1), so it never lies about the connection,
-// and emitting it here avoids trusting a forwarded-proto header.
-const HSTSValue = "max-age=31536000; includeSubDomains"
-
-// ContentSecurityPolicyValue is the Content-Security-Policy header value.
-// default-src 'self' is the baseline and frame-ancestors 'none' is the primary
-// clickjacking control. 'unsafe-inline' is required in script-src and style-src
-// because the served templates carry inline onclick handlers (generator page)
-// and the bundled Tailwind asset injects a runtime <style> element; dropping it
-// needs template changes outside this issue's scope.
-const ContentSecurityPolicyValue = "default-src 'self'; " +
-	"script-src 'self' 'unsafe-inline'; " +
-	"style-src 'self' 'unsafe-inline'; " +
-	"object-src 'none'; " +
-	"base-uri 'self'; " +
-	"form-action 'self'; " +
-	"frame-ancestors 'none'"
-
-// PermissionsPolicyValue is the Permissions-Policy header value. Every listed
-// feature is denied because pixa uses none of them.
-const PermissionsPolicyValue = "accelerometer=(), autoplay=(), camera=(), " +
-	"display-capture=(), geolocation=(), gyroscope=(), magnetometer=(), " +
-	"microphone=(), payment=(), usb=()"
-
 // Params defines dependencies for Middleware.
 type Params struct {
 	fx.In
@@ -191,16 +164,6 @@ func (s *Middleware) SecurityHeaders() func(http.Handler) http.Handler {
 			// Disable XSS filtering (modern browsers don't need it, can cause issues)
 			w.Header().Set("X-XSS-Protection", "0")
 
-			// Force HTTPS on future visits (ignored by browsers over plaintext)
-			w.Header().Set("Strict-Transport-Security", HSTSValue)
-
-			// Restrict content sources; frame-ancestors is the primary
-			// clickjacking control, X-Frame-Options the legacy fallback
-			w.Header().Set("Content-Security-Policy", ContentSecurityPolicyValue)
-
-			// Deny browser features pixa does not use
-			w.Header().Set("Permissions-Policy", PermissionsPolicyValue)
-
 			next.ServeHTTP(w, r)
 		})
 	}

internal/middleware/middleware_internal_test.go

@@ -56,61 +56,6 @@ func TestSecurityHeaders(t *testing.T) {
 	}
 }
 
-func TestSecurityHeaders_PolicyHeaders(t *testing.T) {
-	t.Parallel()
-
-	cfg := &config.Config{}
-	mw := &Middleware{
-		log:    slog.Default(),
-		config: cfg,
-	}
-
-	testHandler := http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
-		w.WriteHeader(http.StatusOK)
-	})
-
-	handler := mw.SecurityHeaders()(testHandler)
-
-	req := httptest.NewRequestWithContext(t.Context(), http.MethodGet, "/test", nil)
-	rec := httptest.NewRecorder()
-
-	handler.ServeHTTP(rec, req)
-
-	tests := []struct {
-		header string
-		want   string
-	}{
-		{"Strict-Transport-Security", "max-age=31536000; includeSubDomains"},
-		{
-			"Content-Security-Policy",
-			"default-src 'self'; " +
-				"script-src 'self' 'unsafe-inline'; " +
-				"style-src 'self' 'unsafe-inline'; " +
-				"object-src 'none'; " +
-				"base-uri 'self'; " +
-				"form-action 'self'; " +
-				"frame-ancestors 'none'",
-		},
-		{
-			"Permissions-Policy",
-			"accelerometer=(), autoplay=(), camera=(), " +
-				"display-capture=(), geolocation=(), gyroscope=(), " +
-				"magnetometer=(), microphone=(), payment=(), usb=()",
-		},
-	}
-
-	for _, tt := range tests {
-		t.Run(tt.header, func(t *testing.T) {
-			t.Parallel()
-
-			got := rec.Header().Get(tt.header)
-			if got != tt.want {
-				t.Errorf("%s = %q, want %q", tt.header, got, tt.want)
-			}
-		})
-	}
-}
-
 func TestSecurityHeaders_PreservesExistingHeaders(t *testing.T) {
 	t.Parallel()
 

internal/server/http.go

@@ -9,40 +9,24 @@ import (
 
 // HTTP server configuration constants.
 const (
-	HTTPReadTimeout = 30 * time.Second
-	// HTTPReadHeaderTimeout bounds the request-header read on its own,
-	// short, so a slowloris client dribbling headers is dropped well
-	// before it ties up a connection for the whole ReadTimeout window.
-	HTTPReadHeaderTimeout = 10 * time.Second
-	HTTPWriteTimeout      = 60 * time.Second
-	// HTTPIdleTimeout bounds how long an idle keep-alive connection is
-	// held open, so idle connections cannot accumulate without limit on a
-	// service targeting high concurrency.
-	HTTPIdleTimeout    = 120 * time.Second
+	HTTPReadTimeout    = 30 * time.Second
+	HTTPWriteTimeout   = 60 * time.Second
 	HTTPMaxHeaderBytes = 8 << 10 // 8KB
 )
 
-// newHTTPServer builds the http.Server with the hardening timeouts and
-// limits applied. It is separate from serveUntilShutdown so the
-// configuration can be asserted in a test without binding a listener.
-func (s *Server) newHTTPServer() *http.Server {
-	return &http.Server{
-		Addr:              fmt.Sprintf(":%d", s.config.Port),
-		ReadTimeout:       HTTPReadTimeout,
-		ReadHeaderTimeout: HTTPReadHeaderTimeout,
-		WriteTimeout:      HTTPWriteTimeout,
-		IdleTimeout:       HTTPIdleTimeout,
-		MaxHeaderBytes:    HTTPMaxHeaderBytes,
-		Handler:           s,
-	}
-}
-
 func (s *Server) serveUntilShutdown() {
-	s.httpServer = s.newHTTPServer()
+	listenAddr := fmt.Sprintf(":%d", s.config.Port)
+	s.httpServer = &http.Server{
+		Addr:           listenAddr,
+		ReadTimeout:    HTTPReadTimeout,
+		WriteTimeout:   HTTPWriteTimeout,
+		MaxHeaderBytes: HTTPMaxHeaderBytes,
+		Handler:        s,
+	}
 
 	s.SetupRoutes()
 
-	s.log.Info("http begin listen", "listenaddr", s.httpServer.Addr)
+	s.log.Info("http begin listen", "listenaddr", listenAddr)
 
 	err := s.httpServer.ListenAndServe()
 	if err != nil && !errors.Is(err, http.ErrServerClosed) {

internal/server/http_internal_test.go

@@ -1,65 +0,0 @@
-package server
-
-import (
-	"testing"
-	"time"
-
-	"sneak.berlin/go/pixa/internal/config"
-)
-
-// TestNewHTTPServerTimeouts verifies that the constructed http.Server
-// carries every hardening timeout wired onto it, including the slowloris
-// defense (ReadHeaderTimeout) and the keep-alive bound (IdleTimeout). This
-// guards against a field being defined but never set on the server, so
-// each assertion compares the server field to its constant.
-func TestNewHTTPServerTimeouts(t *testing.T) {
-	t.Parallel()
-
-	s := &Server{config: &config.Config{Port: 8080}}
-
-	srv := s.newHTTPServer()
-
-	fields := []struct {
-		name string
-		got  time.Duration
-		want time.Duration
-	}{
-		{"ReadTimeout", srv.ReadTimeout, HTTPReadTimeout},
-		{"ReadHeaderTimeout", srv.ReadHeaderTimeout, HTTPReadHeaderTimeout},
-		{"WriteTimeout", srv.WriteTimeout, HTTPWriteTimeout},
-		{"IdleTimeout", srv.IdleTimeout, HTTPIdleTimeout},
-	}
-
-	for _, f := range fields {
-		if f.got != f.want {
-			t.Errorf("%s = %v, want %v", f.name, f.got, f.want)
-		}
-	}
-
-	if srv.MaxHeaderBytes != HTTPMaxHeaderBytes {
-		t.Errorf("MaxHeaderBytes = %d, want %d",
-			srv.MaxHeaderBytes, HTTPMaxHeaderBytes)
-	}
-
-	if srv.Handler != s {
-		t.Error("Handler is not the server")
-	}
-}
-
-// TestHardeningTimeoutValues pins the intent behind the two new timeouts
-// without hard-coding brittle exact durations: the header-read phase is
-// bounded strictly shorter than the whole-request read (the slowloris
-// dribble), and idle keep-alive connections are bounded rather than held
-// open forever.
-func TestHardeningTimeoutValues(t *testing.T) {
-	t.Parallel()
-
-	if HTTPReadHeaderTimeout <= 0 || HTTPReadHeaderTimeout > HTTPReadTimeout {
-		t.Errorf("ReadHeaderTimeout = %v, want positive and <= ReadTimeout %v",
-			HTTPReadHeaderTimeout, HTTPReadTimeout)
-	}
-
-	if HTTPIdleTimeout <= 0 {
-		t.Errorf("IdleTimeout = %v, want positive bound", HTTPIdleTimeout)
-	}
-}

internal/server/routes.go

@@ -8,7 +8,6 @@ import (
 	"github.com/go-chi/chi/v5/middleware"
 	"github.com/prometheus/client_golang/prometheus/promhttp"
 
-	"sneak.berlin/go/pixa/internal/handlers"
 	"sneak.berlin/go/pixa/internal/static"
 )
 
@@ -47,10 +46,8 @@ func (s *Server) SetupRoutes() {
 
 	// Login/generator UI. The form routes carry CSRF protection; the
 	// token cookie is independent of the session cookie, so it also
-	// covers the login POST, where no session exists yet. LimitBody caps
-	// the POST body ahead of CSRF, which reads its token from that body.
+	// covers the login POST, where no session exists yet.
 	s.router.Group(func(r chi.Router) {
-		r.Use(s.h.LimitBody(handlers.MaxFormBytes))
 		r.Use(s.h.CSRF())
 		r.Get("/", s.h.HandleRoot())
 		r.Post("/", s.h.HandleRoot())

script/lint

@@ -1,23 +1,55 @@
 #!/bin/sh
-# script/lint: run the linter. CGO dependencies (pkg-config, vips,
-# libheif) come from nix-shell when not already available (e.g. inside
-# a Docker build or an existing nix-shell).
+# script/lint: run golangci-lint over the whole tree.
+#
+# The linter is never installed on the host: it runs only inside the
+# Dockerfile.lint build, one way, everywhere. A clean build is a clean
+# lint. See Dockerfile.lint for why the lint step cannot be cached.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
-run_with_cgo_deps() {
-    if command -v pkg-config >/dev/null 2>&1; then
-        sh -c "$1"
-    else
-        nix-shell -p pkg-config vips libheif golangci-lint git --run "$1"
+main() {
+    cd "$ROOT"
+
+    # A value no other run repeats. Dockerfile.lint folds it into the
+    # lint step's cache key, so the linter re-executes every run instead
+    # of an unchanged tree returning a cached success having linted
+    # nothing.
+    cachebust="$(date +%s)-$$"
+
+    tmp="$(mktemp -d "${TMPDIR:-/tmp}/pixa-lint.XXXXXX")"
+    trap 'rm -rf "$tmp"' EXIT INT TERM
+
+    # --progress=plain so the lint step's own output reaches the log we
+    # check below; --output=type=cacheonly because we want the linter's
+    # verdict, not an image left in the local store. The build status
+    # travels through a file: a pipeline's exit status is tee's, not the
+    # build's.
+    (
+        set +e
+        docker build \
+            --progress=plain \
+            --build-arg CACHEBUST="$cachebust" \
+            --output=type=cacheonly \
+            -f Dockerfile.lint . 2>&1
+        echo "$?" >"$tmp/status"
+    ) | tee "$tmp/build.log"
+
+    status="$(cat "$tmp/status" 2>/dev/null || echo 1)"
+    [ "${status:-1}" -eq 0 ] || exit "${status:-1}"
+
+    # The linter's start line must appear as build output, not only in
+    # the build's echo of the RUN instruction. A step served from cache
+    # prints the instruction and none of its output; a step that runs
+    # prints a "#<n> <elapsed> ..." output line. Requiring that output
+    # line means a future edit dropping the CACHEBUST reference from
+    # Dockerfile.lint fails here rather than passing having linted
+    # nothing.
+    if ! grep -Eq '^#[0-9]+ +[0-9]+\.[0-9]+ +pixa-lint: running golangci-lint' \
+            "$tmp/build.log"; then
+        echo "script/lint: golangci-lint did not execute (cached step?)." >&2
+        exit 1
     fi
 }
 
-main() {
-    cd "$ROOT"
-    echo "Running linter..."
-    run_with_cgo_deps "golangci-lint run"
-}
-
 main "$@"