Run all linting in Docker via Dockerfile.lint (closes #188)

golangci-lint now runs only in Docker. New Dockerfile.lint (pinned
golangci-lint v2.12.2) COPYs the tree and runs the linter as a build
step; script/lint just builds it. A GATE_RUN build arg differs every
run, so the lint layer always executes -- a cached build would exit 0
having linted nothing.

config verify is deliberately omitted: it fetches its JSON schema over
an unpinned live HTTPS call, which REPO_POLICIES.md forbids.

script/bootstrap no longer installs golangci-lint (goimports kept). The
main Dockerfile lint stage now invokes golangci-lint directly rather
than make lint, so building it is not docker-in-docker.

Model: opus-4-8

script/bootstrap

@@ -3,20 +3,15 @@
 # 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). goimports is installed with `go install` at a pinned
+# make, or go). goimports is installed with `go install` at a pinned
 # version (integrity via the Go module checksum database) into
-# /usr/local/bin so it is on PATH.
+# /usr/local/bin so it is on PATH. The linter is not installed here: it
+# runs only in Docker via script/lint, so docker is its sole prerequisite.
 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"
+# Pinned versions. Never "latest"; exact versions only.
 # golang.org/x/tools goimports, 2026-08-13. v0.49.0 requires Go 1.25 (matches
 # go.mod); v0.50.0 needs Go 1.26. Integrity via the Go module checksum database.
 GOIMPORTS_VERSION="v0.49.0"
@@ -61,56 +56,10 @@ 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
-}
-
 # goimports is not packaged uniformly across nix/apt/brew/apk, so install it
 # with `go install` at a pinned version and place the binary in /usr/local/bin
-# so it is on PATH regardless of shell config, as the golangci-lint release
-# install does. Requires go, which main installs first.
+# so it is on PATH regardless of shell config. Requires go, which main
+# installs first.
 ensure_goimports() {
     if ! missing goimports; then return 0; fi
     detect_pkgmgr
@@ -127,11 +76,18 @@ 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
     ensure_goimports
 
+    # The linter runs only in Docker (script/lint). Warn, don't fail: the
+    # rest of the repo works without it.
+    if missing docker; then
+        echo "bootstrap: WARNING: docker not found; make lint and" >&2
+        echo "bootstrap: make check require it. Install docker to run" >&2
+        echo "bootstrap: the linter." >&2
+    fi
+
     go mod download
 
     echo "bootstrap complete"