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.

Dockerfile

@@ -1,13 +1,9 @@
-# Build stage
-# golang 1.25-alpine, 2026-02-28
-FROM golang@sha256:f6751d823c26342f9506c03797d2527668d095b0a15f1862cddb4d927a7a4ced AS builder
-
-RUN apk add --no-cache git make gcc musl-dev binutils-gold
-
-# golangci-lint v2.12.2, 2026-08-07
-RUN go install github.com/golangci/golangci-lint/v2/cmd/golangci-lint@c0d3ddc9cf3faa61a4e378e879ece580256d76e5
-# goimports v0.42.0
-RUN go install golang.org/x/tools/cmd/goimports@009367f5c17a8d4c45a961a3a509277190a9a6f0
+# Lint stage - fast feedback on lint issues, before the build starts.
+# The linter is invoked directly rather than through `make lint`: that
+# target shells out to `docker build -f Dockerfile.lint`, and there is
+# no docker daemon inside a docker build.
+# golangci/golangci-lint:v2.12.2 (Debian-based), 2026-08-10
+FROM golangci/golangci-lint:v2.12.2@sha256:5cceeef04e53efe1470638d4b4b4f5ceefd574955ab3941b2d9a68a8c9ad5240 AS lint
 
 WORKDIR /src
 COPY go.mod go.sum ./
@@ -15,8 +11,26 @@ RUN go mod download
 
 COPY . .
 
-# Run all checks - build fails if any check fails
-RUN make check
+RUN make fmt-check
+RUN golangci-lint run --config .golangci.yml ./...
+
+# Build stage
+# golang 1.25-alpine, 2026-02-28
+FROM golang@sha256:f6751d823c26342f9506c03797d2527668d095b0a15f1862cddb4d927a7a4ced AS builder
+
+RUN apk add --no-cache git make gcc musl-dev binutils-gold
+
+# Force BuildKit to run the lint stage before proceeding
+COPY --from=lint /src/go.sum /dev/null
+
+WORKDIR /src
+COPY go.mod go.sum ./
+RUN go mod download
+
+COPY . .
+
+# Run the tests - build fails if any test fails
+RUN make test
 
 # Build the binary
 RUN make build