build: run all linting in Docker via Dockerfile.lint (closes #134)

golangci-lint is no longer installed or run on the host. script/lint is
now a thin wrapper that builds the new root Dockerfile.lint, which COPYs
the repo into the digest-pinned golangci/golangci-lint:v2.12.2 image and
lints as a build step, so a successful build is a clean lint. This works
even where the docker daemon is remote and bind mounts are impossible.

Dockerfile.lint is split into a deps stage (base image, go mod download)
and a lint stage (source copy, linter run). script/lint passes
--no-cache-filter=lint so the lint stage executes on every invocation:
caching is explicitly waived for linting, and a cached build lints
nothing. The deps stage stays cached and no global cache invalidation is
performed. --progress=plain keeps the linter's own output visible.

golangci-lint config verify is deliberately omitted: it fetches its JSON
schema over a live, unpinned HTTPS call, which would make linting
network-dependent and defeat hash-pinning.

script/bootstrap no longer installs golangci-lint and warns instead when
docker is absent. The goimports install stays, since script/fmt and
script/fmt-check still run it on the host.

The root Dockerfile ran make check in its builder stage, which would now
recurse into script/lint and shell out to docker build with no daemon
available. It gains its own lint stage on the same pinned image, invoked
directly, with the builder depending on it via COPY --from=lint and
running make fmt-check, make test and make build.

script/bootstrap

@@ -3,15 +3,16 @@
 # 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.
-# golangci-lint and goimports are installed via `go install` at the same
-# pinned commits the Dockerfile uses (never "latest").
+# goimports is installed via `go install` at a pinned commit (never
+# "latest") because script/fmt and script/fmt-check run it on the host.
+# The linter is NOT installed here: golangci-lint runs via docker only
+# (script/lint), pinned by image digest, so its only prerequisite is a
+# working docker.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
-# Pinned versions, 2026-08-07 (same pins as the Dockerfile)
-# golangci-lint v2.12.2
-GOLANGCI_LINT_REF="github.com/golangci/golangci-lint/v2/cmd/golangci-lint@c0d3ddc9cf3faa61a4e378e879ece580256d76e5"
+# Pinned version, 2026-08-07 (same pin as the Dockerfile)
 # goimports v0.42.0
 GOIMPORTS_REF="golang.org/x/tools/cmd/goimports@009367f5c17a8d4c45a961a3a509277190a9a6f0"
 
@@ -69,11 +70,18 @@ main() {
     if missing make; then pkg_install gnumake make make make; fi
     if missing go; then pkg_install go golang go go; fi
 
-    # Lint/format tools, pinned via go install (installs into
+    # Format tools, pinned via go install (installs into
     # "$(go env GOPATH)/bin"; ensure that is on your PATH).
-    if missing golangci-lint; then go install "$GOLANGCI_LINT_REF"; fi
     if missing goimports; then go install "$GOIMPORTS_REF"; fi
 
+    # Linting runs via docker only (script/lint). Warn, don't fail:
+    # everything except `make lint` works without it.
+    if missing docker; then
+        echo "bootstrap: WARNING: docker not found; make lint and" >&2
+        echo "bootstrap: make docker require it. Install docker to" >&2
+        echo "bootstrap: run the linter." >&2
+    fi
+
     go mod download
 
     echo "bootstrap complete"

script/lint

@@ -1,12 +1,28 @@
 #!/bin/sh
-# script/lint: run the linter.
+# script/lint: run the linter. golangci-lint is never installed or run
+# on the host: it runs via docker only, one way, everywhere. This
+# builds Dockerfile.lint, which COPYs the repo into the digest-pinned
+# golangci-lint image and lints as a build step, so a successful build
+# means a clean lint.
+#
+# --no-cache-filter=lint forces the lint stage (source copy + linter
+# run) to execute on every invocation. Without it an unchanged tree
+# returns success in well under a second having linted nothing. The
+# deps stage (base image + go mod download) stays cached, and no global
+# cache invalidation is performed. --progress=plain keeps the linter's
+# own output visible.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
 main() {
     cd "$ROOT"
-    golangci-lint run --config .golangci.yml ./...
+    docker build \
+        --progress=plain \
+        --no-cache-filter=lint \
+        --target lint \
+        -f Dockerfile.lint \
+        .
 }
 
 main "$@"