diff --git a/Dockerfile b/Dockerfile
index 8779e06..8580a63 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -5,8 +5,25 @@ WORKDIR /src
 COPY go.mod go.sum ./
 RUN go mod download
 COPY . .
-RUN make fmt-check
-RUN make lint
+
+# Cache-buster for the gate layers, and only for them. Docker
+# invalidates COPY only when the copied content changes, so on an
+# unchanged tree the gates below would be served from cache and the
+# build would exit 0 having run nothing. script/cibuild and
+# script/docker pass a fresh CHECK_EPOCH on every invocation.
+#
+# Two properties this depends on. ARG is per-stage, so the build stage
+# below declares it again; one declaration here would leave that
+# stage's gate cacheable. And each gate RUN must reference the value,
+# because BuildKit hashes the expanded command: a declared but
+# unreferenced ARG invalidates nothing.
+#
+# It sits below the dependency layers deliberately. Everything above it
+# (the pinned base image, go mod download) keeps its cache; only the
+# gates go cold.
+ARG CHECK_EPOCH
+RUN echo "gate fmt-check, epoch ${CHECK_EPOCH}" && make fmt-check
+RUN echo "gate lint, epoch ${CHECK_EPOCH}" && make lint
 
 # Build stage
 # golang:1.25-alpine, 2026-07-23
@@ -40,7 +57,12 @@ USER builder
 # Fail the build unless the branch is green. Runs as non-root so the
 # permission-denied test paths are exercised legitimately (root would
 # bypass the chmod(0) the tests rely on).
-RUN make check
+#
+# Second per-stage declaration of the gate cache-buster; see the lint
+# stage above for why one is not enough. It is placed after USER so the
+# drop to the unprivileged user still happens before the checks run.
+ARG CHECK_EPOCH
+RUN echo "gate check, epoch ${CHECK_EPOCH}" && make check
 
 RUN make build
 
diff --git a/TODO.md b/TODO.md
index c23b4f4..795ad2a 100644
--- a/TODO.md
+++ b/TODO.md
@@ -29,6 +29,47 @@
 
 # Completed Steps
 
+- bust the Docker layer cache for the gate steps, so `script/cibuild`
+  and `script/docker` cannot report a green they did not earn
+  (2026-08-09, branch `cibuild-cache-bust`, closes #32): both scripts
+  were bare `docker build` invocations with no cache control, and the
+  `Dockerfile` copies the tree before running its gates, so on an
+  unchanged tree Docker served those layers from cache and the build
+  exited 0 having executed nothing. That is not hypothetical here —
+  every merge this repo has done is a non-fast-forward merge of an
+  undiverged branch, so each merge commit's tree is byte-identical to
+  the branch head's and each merge CI run was almost certainly a full
+  cache hit; and PR #31's reviewer found `make docker` returning
+  success as a 17-layer cache hit, catching it only by being
+  suspicious. The fix is `ARG CHECK_EPOCH` with the scripts passing
+  `--build-arg CHECK_EPOCH="$(date +%s)"`. Two details make or break
+  it. `ARG` is scoped per stage and this `Dockerfile` has three gates
+  across two — `make fmt-check` and `make lint` in the lint stage,
+  `make check` in the build stage — so a single declaration would have
+  left one stage silently cacheable; it is declared in both. And
+  BuildKit hashes the expanded command, not the declaration, so a
+  declared-but-unreferenced `ARG` invalidates nothing: each gate `RUN`
+  echoes the epoch, which also puts the value in the build log as
+  evidence the layer really ran. Placement is below the dependency
+  layers on purpose — a build that goes cold every time would be a
+  different bug, not a fix. Verified by running each script twice back
+  to back on an unchanged tree under `BUILDKIT_PROGRESS=plain`: all
+  three gates executed on all four runs, each with a fresh epoch in
+  the log (`script/cibuild` 78.8s then 61.1s; `script/docker` 61.1s
+  then 53.4s), and twelve steps were still served `CACHED` in the
+  steady state — both `go mod download`s, `apk add`, `adduser`, the
+  `chown`, every `go.mod`/`go.sum` and source copy, the linter copy
+  out of the lint stage, and the binary copy into the runtime stage.
+  The lint stage still gates the build stage: with a deliberate
+  `unused` finding planted in the tree, the build failed at
+  `make lint` in 36.1s and the build-stage `make check` never started.
+  The build stage also still drops to the unprivileged `builder` user
+  before `make check`, which the suite depends on rather than merely
+  prefers: forcing the same image to run the tests as root fails
+  `TestScanHardlinkRunFailsTogether`, because root reads straight
+  through the `chmod(0)` the test uses to prove hard links are read
+  once. This is the local fix only; propagating it to the canonical
+  templates is `prompts` #26
 - check the installed golangci-lint version in `script/bootstrap`
   instead of only its presence (2026-08-09, branch
   `bootstrap-version-check`, closes #24): `missing golangci-lint` meant
diff --git a/script/cibuild b/script/cibuild
index 7d63316..e71140a 100755
--- a/script/cibuild
+++ b/script/cibuild
@@ -1,14 +1,23 @@
 #!/bin/sh
-# script/cibuild: run the CI build. The Dockerfile runs make check (via
-# script/check), so a successful build implies all checks pass. The
-# Gitea workflow runs this on push.
+# script/cibuild: run the CI build. The Dockerfile runs make fmt-check,
+# make lint and make check (via the script/ entrypoints) as build steps,
+# so a successful build implies all checks pass. The Gitea workflow runs
+# this on push.
+#
+# That implication holds only because of CHECK_EPOCH. A COPY layer is
+# invalidated by changed content, and a merge commit's tree is
+# byte-identical to the branch head it merges, so without a fresh value
+# here Docker serves the gate layers from cache and the build reports a
+# green it never earned. Passing the current epoch invalidates the gate
+# layers on every run while leaving the pinned base images and
+# go mod download cached; see the Dockerfile for the placement.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
 main() {
     cd "$ROOT"
-    docker build .
+    docker build --build-arg CHECK_EPOCH="$(date +%s)" .
 }
 
 main "$@"
diff --git a/script/docker b/script/docker
index 9b9ea86..b5de765 100755
--- a/script/docker
+++ b/script/docker
@@ -1,6 +1,14 @@
 #!/bin/sh
 # script/docker: build the Docker image tagged with the project name.
-# Identical in all repos; the tag comes from script/projectname.
+# The tag comes from script/projectname.
+#
+# CHECK_EPOCH is passed for the same reason script/cibuild passes it:
+# without it Docker serves the Dockerfile's gate layers from cache on an
+# unchanged tree and this exits 0 having run neither the lint stage nor
+# the builder stage's make check. This is the gate a developer or
+# reviewer runs by hand, so a cached pass here is the most misleading
+# result the repo can produce. Dependency layers sit above the ARG and
+# stay cached.
 set -eu
 
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"
@@ -8,7 +16,10 @@ ROOT="$(cd "$SCRIPT_DIR/.." && pwd -P)"
 
 main() {
     cd "$ROOT"
-    docker build -t "$("$SCRIPT_DIR/projectname")" .
+    docker build \
+        --build-arg CHECK_EPOCH="$(date +%s)" \
+        -t "$("$SCRIPT_DIR/projectname")" \
+        .
 }
 
 main "$@"