diff --git a/Dockerfile b/Dockerfile
index 020f59f..4cfba6d 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -1,35 +1,43 @@
-# 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, 2026-10-05
-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 phase
+# golangci/golangci-lint:v2.12.2, 2026-10-05
+FROM golangci/golangci-lint@sha256:5cceeef04e53efe1470638d4b4b4f5ceefd574955ab3941b2d9a68a8c9ad5240 AS lint
 WORKDIR /src
 COPY go.mod go.sum ./
 RUN go mod download
+COPY . .
+RUN golangci-lint run --config .golangci.yml ./...
 
+# Test phase. -race needs cgo and so a C compiler, which the Debian Go
+# image ships and the alpine one does not. The tests run as an
+# unprivileged user: root can read a file with mode 0000, so the
+# permission test would fail.
+# golang:1.25.7-trixie, 2026-10-06
+FROM golang@sha256:2b174ffcf56c7ad0c47d30d2630693265639ddf2a5141149c2da34db921791b4 AS test
+RUN useradd --create-home testuser
+USER testuser
+WORKDIR /src
+COPY go.mod go.sum ./
+RUN go mod download
+COPY . .
+RUN go test -timeout 90s -race -cover ./... || \
+    { echo "--- Rerunning with -v for details ---"; \
+      go test -timeout 90s -race -v ./...; exit 1; }
+
+# Build stage. Nothing is wanted from either phase above; the copies
+# are what make BuildKit build them first, so this stage cannot run
+# unless lint and test passed.
+# golang 1.25-alpine, 2026-02-28
+FROM golang@sha256:f6751d823c26342f9506c03797d2527668d095b0a15f1862cddb4d927a7a4ced AS builder
+COPY --from=lint /src/go.sum /dev/null
+COPY --from=test /src/go.sum /dev/null
+RUN apk add --no-cache git make
+# A tar-stream context keeps the sender's file owners, which git refuses.
+RUN git config --system --add safe.directory /src
+WORKDIR /src
+COPY go.mod go.sum ./
+RUN go mod download
 COPY . .
 
-# Run the checks as an unprivileged user. Root bypasses file mode bits, which
-# would make the permission tests (expecting EACCES on a 0000 file) spuriously
-# pass with no error. Caches live under /tmp (world-writable) so the user needs
-# no home directory of its own.
-ENV GOCACHE=/tmp/gocache
-ENV XDG_CACHE_HOME=/tmp/xdgcache
-RUN adduser -D -u 1000 builder && chown -R builder:builder /src /go
-USER builder
-
-# Run all checks - build fails if any check fails
-RUN make check
-
-# Build the binary (still as the unprivileged user: it owns /src, so git VCS
-# stamping sees consistent ownership).
-#
 # The version stamped into the binary: the VERSION build argument when one
 # is given, otherwise `git describe --tags --always` of the .git the build
 # context carries: the tag on a tagged commit, tag-N-gHASH on a commit after
diff --git a/README.md b/README.md
index dcfb598..e5ad7d3 100644
--- a/README.md
+++ b/README.md
@@ -95,7 +95,9 @@ Future improvements under consideration:
 
 * Author & maintainer: **sneak** – <sneak@sneak.berlin>
 * Issues / PRs: <https://git.eeqj.de/sneak/attrsum/>
-* Code must pass `go vet`, `go test ./...`, and `go fmt`.
+* Code must pass `make check`, which runs the tests and golangci-lint as
+  phases of the `Dockerfile` (Docker is required) and checks formatting
+  with `gofmt`.
 * No CLA; contributions are under WTFPL v2.
 
 ---
diff --git a/TODO.md b/TODO.md
index bc56565..4d9a89a 100644
--- a/TODO.md
+++ b/TODO.md
@@ -17,13 +17,19 @@ have landed since the tag.
 
 # Next Step
 
-Policy scaffold commit: add LICENSE, REPO_POLICIES.md, .editorconfig,
-.golangci.yml, and a comprehensive .gitignore (currently only the
-attrsum binary), and extend the Makefile (only test/build/clean/try
-today) with lint, fmt, fmt-check, check, and hooks targets.
+Re-vendor the canonical files from `sneak/prompts` at `dd4027b`
+(https://git.eeqj.de/sneak/attrsum/issues/13): add `REPO_POLICIES.md`
+and `.editorconfig`, refresh `.gitignore` (only the `attrsum` binary
+today), `.dockerignore`, `.gitea/workflows/check.yml` and
+`.golangci.yml`, and move the lint phase to golangci-lint v2.14.0.
 
 # Completed Steps
 
+* 2026-10-06: lint and test run as phases of the `Dockerfile`, and the
+    build stage depends on both; `script/lint` and `script/test` each
+    build their phase with `--no-cache`; `script/cibuild` bootstraps,
+    runs `script/check`, then builds the image; golangci-lint is no
+    longer installed on the host
 * 2026-10-05: golangci-lint settings take effect: canonical
     `.golangci.yml` (v2 layout, settings under `linters.settings`),
     golangci-lint pinned at v2.12.2 in `Dockerfile` and
@@ -50,11 +56,12 @@ today) with lint, fmt, fmt-check, check, and hooks targets.
 
 # Future Steps
 
-* Add .gitea/workflows/check.yml
 * Restructure README.md into the standard sections: Description,
-    Getting Started, Rationale, Design, TODO, License, Author (Getting
-    Started, Why?, TODO, License exist; Description, Design, Author are
-    missing)
+    Getting Started, Entrypoints, Rationale, Design, TODO, License,
+    Author (Getting Started, Why?, TODO, License exist; Description,
+    Entrypoints, Design, Author are missing)
+* Add a `LICENSE` file matching the README's WTFPL v2; sneak's to add,
+    not an agent's
 * Tag a patch release to ship the 2026-02-02 correctness fixes
 * Dry-run mode (--dry-run, -n): show what would be done without making
     changes (from README TODO)
diff --git a/script/bootstrap b/script/bootstrap
index 1d6577e..f7f985f 100755
--- a/script/bootstrap
+++ b/script/bootstrap
@@ -3,15 +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.
-# 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"). The linter is not installed: it runs only as the lint phase
+# of the Dockerfile.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
-# Pinned versions, 2026-10-05 (same pins as the Dockerfile)
-# golangci-lint v2.12.2
-GOLANGCI_LINT_REF="github.com/golangci/golangci-lint/v2/cmd/golangci-lint@c0d3ddc9cf3faa61a4e378e879ece580256d76e5"
+# Pinned versions, 2026-10-05
 # goimports v0.42.0
 GOIMPORTS_REF="golang.org/x/tools/cmd/goimports@009367f5c17a8d4c45a961a3a509277190a9a6f0"
 
@@ -69,9 +68,8 @@ 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 tool, 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
 
     go mod download
diff --git a/script/check b/script/check
index 3e1778c..92875f7 100755
--- a/script/check
+++ b/script/check
@@ -1,6 +1,8 @@
 #!/bin/sh
 # script/check: run all checks (test, lint, fmt-check). Our own
-# extension to scripts-to-rule-them-all. Must not modify any files.
+# extension to scripts-to-rule-them-all. test and lint are Docker
+# phases; fmt-check is native, because a formatter writes the working
+# tree. Must not modify any files.
 set -eu
 
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"
diff --git a/script/cibuild b/script/cibuild
index 966f51d..d8d3200 100755
--- a/script/cibuild
+++ b/script/cibuild
@@ -1,13 +1,28 @@
 #!/bin/sh
-# script/cibuild: run the CI build. The Dockerfile runs make check, so
-# a successful build implies all checks pass.
+# script/cibuild: run the CI build. It bootstraps first: a CI runner
+# checks out and runs this and nothing else, and script/fmt-check runs
+# the formatter on the host, which a pristine checkout cannot do.
+# --no-cache for the same reason as script/docker: the gate phases the
+# final stage depends on are RUN steps, and a cached one is a check that
+# did not run.
 set -eu
 
-ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"
+ROOT="$(cd "$SCRIPT_DIR/.." && pwd -P)"
 
 main() {
     cd "$ROOT"
-    docker build .
+    "$SCRIPT_DIR/bootstrap"
+    "$SCRIPT_DIR/check"
+    # Own line: a failing command substitution inside an argument does
+    # not trip `set -e`, so the inline form degrades silently to an
+    # empty constant. The VERSION build argument takes precedence over
+    # the version a build stage derives from the .git in the context.
+    version="$(git describe --tags --always --dirty 2>/dev/null || true)"
+    [ -n "$version" ] || version="unknown"
+    docker build --no-cache \
+        --build-arg VERSION="$version" \
+        -t "$("$SCRIPT_DIR/projectname")" .
 }
 
 main "$@"
diff --git a/script/lint b/script/lint
index 8017180..2d8b075 100755
--- a/script/lint
+++ b/script/lint
@@ -1,12 +1,23 @@
 #!/bin/sh
-# script/lint: run the linter.
+# script/lint: run the linter. Linting is a phase of the Dockerfile and
+# this builds that phase alone; the linter is never installed or run on
+# a developer host, where a shared result cache and a host-global lock
+# make its answer untrustworthy.
+#
+# The phase is not the last stage in the file, so it is built only when
+# --target names it. --no-cache because a cached lint layer is a lint
+# that did not run. The tag makes each build replace the previous image
+# instead of leaving a dangling one behind.
 set -eu
 
-ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"
+ROOT="$(cd "$SCRIPT_DIR/.." && pwd -P)"
 
 main() {
     cd "$ROOT"
-    golangci-lint run --config .golangci.yml ./...
+    docker build --no-cache \
+        --target lint \
+        -t "$("$SCRIPT_DIR/projectname")-lint" .
 }
 
 main "$@"
diff --git a/script/test b/script/test
index 50b1730..cd239f2 100755
--- a/script/test
+++ b/script/test
@@ -1,12 +1,19 @@
 #!/bin/sh
-# script/test: run the test suite.
+# script/test: run the test suite. Testing is a phase of the Dockerfile
+# and this builds that phase alone, on the same terms as script/lint:
+# --target because a phase that is not the last stage is built only when
+# named, --no-cache because a cached test layer is a test that did not
+# run, and a tag so each build replaces the previous image.
 set -eu
 
-ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"
+ROOT="$(cd "$SCRIPT_DIR/.." && pwd -P)"
 
 main() {
     cd "$ROOT"
-    go test -v -race -timeout 30s -cover ./...
+    docker build --no-cache \
+        --target test \
+        -t "$("$SCRIPT_DIR/projectname")-test" .
 }
 
 main "$@"