diff --git a/Dockerfile b/Dockerfile
index 96d4a5e..8451b56 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -1,4 +1,5 @@
 # Lint stage
+# Same image as Dockerfile.lint: change both pins together.
 # golangci/golangci-lint:v2.12.2-alpine, 2026-08-07
 FROM golangci/golangci-lint:v2.12.2-alpine@sha256:91b27804074a0bacea298707f016911e60cf0cdbc6c7bf5ccacb5f0606d18d60 AS lint
 
@@ -13,6 +14,9 @@ RUN go mod download
 # Copy source code
 COPY . .
 
+# Tells script/lint it is inside a container, so it runs the linter.
+ENV container=docker
+
 # Run formatting check and linter
 RUN make fmt-check
 RUN make lint
diff --git a/Dockerfile.lint b/Dockerfile.lint
new file mode 100644
index 0000000..a07d8ca
--- /dev/null
+++ b/Dockerfile.lint
@@ -0,0 +1,34 @@
+# Dockerfile.lint: the container script/lint builds to run golangci-lint,
+# which is never installed on the host. Pinned to the same image as the
+# Dockerfile lint stage: change both pins together, or the two run
+# 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 . .
+
+# Tells script/lint it is inside a container, so it runs the linter.
+ENV container=docker
+
+# script/lint passes a different CACHEBUST on every run, and BuildKit
+# keys every RUN after this ARG on its value, so the lint step always
+# runs instead of returning a cached success that linted nothing.
+#
+# Go's and golangci-lint's caches (/root/.cache, hundreds of MB) go on a
+# tmpfs that is discarded after the step. Written into the layer, they
+# would pile up as build cache on every run, since no later run, with
+# its new CACHEBUST, can reuse that layer.
+ARG CACHEBUST
+RUN --mount=type=tmpfs,target=/root/.cache script/lint
diff --git a/Makefile b/Makefile
index 27001a8..708d5e5 100644
--- a/Makefile
+++ b/Makefile
@@ -10,7 +10,7 @@ ifdef HAS_PKGCONFIG
   NIX_RUN_PREFIX =
   NIX_RUN_SUFFIX =
 else
-  NIX_RUN_PREFIX = nix-shell -p pkg-config vips libheif golangci-lint git --run '
+  NIX_RUN_PREFIX = nix-shell -p pkg-config vips libheif git --run '
   NIX_RUN_SUFFIX = '
 endif
 
diff --git a/README.md b/README.md
index 4c4d859..d3b2b13 100644
--- a/README.md
+++ b/README.md
@@ -207,7 +207,8 @@ them. We provide:
   (bootstrap, then install-precommit)
 - `script/projectname` — output the project name ("pixa")
 - `script/test` — run the test suite
-- `script/lint` — run golangci-lint
+- `script/lint` — run golangci-lint, always in a container (builds
+  `Dockerfile.lint` when run outside one)
 - `script/fmt` — format all code (writes)
 - `script/fmt-check` — check formatting (read-only)
 - `script/check` — run test, lint, and fmt-check
diff --git a/TODO.md b/TODO.md
index 4dc5290..e859d2e 100644
--- a/TODO.md
+++ b/TODO.md
@@ -30,6 +30,18 @@ exhaustion
 
 # Completed Steps
 
+- 2026-09-28 run all linting in Docker via `Dockerfile.lint` +
+  `script/lint` (closes #104): `make lint` calls `script/lint`, the only
+  way the linter is run; inside a container (both Dockerfiles set
+  `container=docker`) it runs `golangci-lint`, anywhere else it builds the
+  hash-pinned `Dockerfile.lint`, whose last step runs `script/lint` again;
+  the `Dockerfile` lint stage runs `make lint`; no host or nix-shell
+  `golangci-lint` path remains (`script/bootstrap` installs no linter);
+  a per-run `CACHEBUST` build-arg keeps the lint step from being served
+  from cache, and a tmpfs mount on that step keeps Go's and
+  golangci-lint's caches out of its layer, so a run leaves no large build
+  cache behind; `golangci-lint config verify` stays out, as it fetches its
+  schema over an unpinned live HTTPS call
 - 2026-09-28 every setting as an environment variable (closes #128, also
   covers #99): each config key can be set by `PIXA_` plus the key in upper
   case (`.` written as `_`), and the port by `PORT`; a variable present in
diff --git a/script/bootstrap b/script/bootstrap
index c1abf08..83d2fc5 100755
--- a/script/bootstrap
+++ b/script/bootstrap
@@ -3,20 +3,14 @@
 # this repo. Idempotent: every install is guarded by a check so already
 # installed tools are skipped. Base tooling comes from nix, apt, brew,
 # or apk (detected in that order); assumes NOTHING is present (not git,
-# make, or go). golangci-lint is packaged in nix, brew, and apk; on apt
-# it is installed from a hash-verified GitHub release archive (never
-# curl | sh). CGO image libraries (pkg-config, vips, libheif) are
-# installed for the govips bindings.
+# make, or go). The linter is never installed on the host: golangci-lint
+# runs only inside a container, Dockerfile.lint or the Dockerfile lint
+# stage (see script/lint). CGO image libraries (pkg-config, vips,
+# libheif) are installed for the govips bindings.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
-# Pinned versions, 2026-08-07. Never "latest"; exact versions only.
-GOLANGCI_LINT_VERSION="2.12.2"
-# sha256 of golangci-lint-2.12.2-linux-<arch>.tar.gz release archives
-GOLANGCI_LINT_SHA256_AMD64="8df580d2670fed8fa984aac0507099af8df275e665215f5c7a2ae3943893a553"
-GOLANGCI_LINT_SHA256_ARM64="44cd40a8c76c86755375adfeea52cfd3533cb43d7bd647771e0ae065e166df3a"
-
 PKGMGR=""
 SUDO=""
 
@@ -57,52 +51,6 @@ missing() {
     ! command -v "$1" >/dev/null 2>&1
 }
 
-# verify_sha256 <file> <expected-hash>
-verify_sha256() {
-    if command -v sha256sum >/dev/null 2>&1; then
-        actual="$(sha256sum "$1" | cut -d' ' -f1)"
-    else
-        actual="$(shasum -a 256 "$1" | cut -d' ' -f1)"
-    fi
-    if [ "$actual" != "$2" ]; then
-        echo "bootstrap: sha256 mismatch for $1" >&2
-        echo "  expected: $2" >&2
-        echo "  actual:   $actual" >&2
-        exit 1
-    fi
-}
-
-# apt has no golangci-lint package: install a pinned release archive
-# from GitHub, verified by hardcoded sha256 (never curl | sh).
-install_golangci_lint_release() {
-    case "$(uname -m)" in
-        x86_64) goarch="amd64"; sha="$GOLANGCI_LINT_SHA256_AMD64" ;;
-        aarch64|arm64) goarch="arm64"; sha="$GOLANGCI_LINT_SHA256_ARM64" ;;
-        *)
-            echo "bootstrap: unsupported architecture $(uname -m)" >&2
-            exit 1
-            ;;
-    esac
-    if missing curl; then pkg_install curl curl curl curl; fi
-    name="golangci-lint-${GOLANGCI_LINT_VERSION}-linux-${goarch}"
-    tmp="$(mktemp -d)"
-    curl -fsSL -o "$tmp/$name.tar.gz" \
-        "https://github.com/golangci/golangci-lint/releases/download/v${GOLANGCI_LINT_VERSION}/${name}.tar.gz"
-    verify_sha256 "$tmp/$name.tar.gz" "$sha"
-    tar -xzf "$tmp/$name.tar.gz" -C "$tmp"
-    $SUDO install -m 0755 "$tmp/$name/golangci-lint" /usr/local/bin/golangci-lint
-    rm -rf "$tmp"
-}
-
-ensure_golangci_lint() {
-    if ! missing golangci-lint; then return 0; fi
-    detect_pkgmgr
-    case "$PKGMGR" in
-        apt) install_golangci_lint_release ;;
-        *) pkg_install golangci-lint golangci-lint golangci-lint golangci-lint ;;
-    esac
-}
-
 # CGO dependencies for govips (image processing)
 ensure_cgo_deps() {
     if missing pkg-config; then
@@ -123,9 +71,8 @@ main() {
     if missing git; then pkg_install git git git git; fi
     if missing make; then pkg_install gnumake make make make; fi
 
-    # Go toolchain and linter
+    # Go toolchain
     if missing go; then pkg_install go golang go go; fi
-    ensure_golangci_lint
 
     # CGO image libraries
     ensure_cgo_deps
diff --git a/script/lint b/script/lint
index 02c75c1..96aa46d 100755
--- a/script/lint
+++ b/script/lint
@@ -1,23 +1,37 @@
 #!/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. This is the only
+# way the linter is run, everywhere; it is never installed on the host.
+#
+# Inside a container it runs the linter. Anywhere else it builds
+# Dockerfile.lint, whose last step runs this script again inside that
+# container.
+#
+# Dockerfile.lint and the Dockerfile lint stage set container=docker
+# (the systemd convention for marking a container) to say where we are.
+# /.dockerenv cannot: it is missing inside build steps, and present on
+# hosts that are themselves containers.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
-run_with_cgo_deps() {
-    if command -v pkg-config >/dev/null 2>&1; then
-        sh -c "$1"
+main() {
+    cd "$ROOT"
+    if [ "${container:-}" = docker ]; then
+        # `golangci-lint config verify` is not run: it fetches its JSON
+        # schema over an unpinned live HTTPS call, which REPO_POLICIES.md
+        # forbids.
+        echo "Running linter..."
+        golangci-lint run --config .golangci.yml ./...
     else
-        nix-shell -p pkg-config vips libheif golangci-lint git --run "$1"
+        # A new CACHEBUST on every run means the lint step is never
+        # served from cache (see Dockerfile.lint). The cacheonly output
+        # leaves no image behind.
+        docker build \
+            --progress=plain \
+            --build-arg CACHEBUST="$(date +%s)-$$" \
+            --output=type=cacheonly \
+            -f Dockerfile.lint .
     fi
 }
 
-main() {
-    cd "$ROOT"
-    echo "Running linter..."
-    run_with_cgo_deps "golangci-lint run"
-}
-
 main "$@"
diff --git a/script/test b/script/test
index e9b4954..c0bcc75 100755
--- a/script/test
+++ b/script/test
@@ -10,7 +10,7 @@ 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"
+        nix-shell -p pkg-config vips libheif git --run "$1"
     fi
 }