Fail a bare docker build instead of serving cached gates (closes #39)

Each Dockerfile stage that runs gates now checks, right after its
ARG CHECK_EPOCH, that the value is not empty, and stops with a message
naming script/cibuild and script/docker. A plain `docker build .` can
no longer report a green from cached gate layers.

script/cibuild and script/docker now append the process id to the
epoch, the form script/lint already uses, so two runs started in the
same second still get different values.

README says both. TODO.md corrects the steady-state CACHED count
recorded for issue 32 from twelve to thirteen.

Model: opus-5-5

Dockerfile

@@ -10,7 +10,9 @@ COPY . .
 # 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.
+# script/docker pass a fresh CHECK_EPOCH on every invocation. A build
+# that passes none, such as a bare `docker build .`, fails at the check
+# right after the ARG instead of quietly serving the gates from cache.
 #
 # Two properties this depends on. ARG is per-stage, so the markdown and
 # build stages below declare it again; one declaration here would leave
@@ -22,6 +24,10 @@ COPY . .
 # (the pinned base image, go mod download) keeps its cache; only the
 # gates go cold.
 ARG CHECK_EPOCH
+RUN if [ -z "${CHECK_EPOCH}" ]; then \
+        echo "CHECK_EPOCH is unset; build via script/cibuild or script/docker" >&2; \
+        exit 1; \
+    fi
 
 # The linter is invoked directly here, not through `make lint`. That
 # target now runs `docker build -f Dockerfile.lint`, and a docker build
@@ -71,9 +77,13 @@ WORKDIR /src
 # Markdown stage: the Markdown half of `make fmt-check`, as a gate.
 FROM prettier AS markdown
 COPY . .
-# Second per-stage declaration of the gate cache-buster; see the lint
-# stage above.
+# Second per-stage declaration of the gate cache-buster and its check;
+# see the lint stage above.
 ARG CHECK_EPOCH
+RUN if [ -z "${CHECK_EPOCH}" ]; then \
+        echo "CHECK_EPOCH is unset; build via script/cibuild or script/docker" >&2; \
+        exit 1; \
+    fi
 RUN echo "gate prettier, epoch ${CHECK_EPOCH}" && \
     prettier --check '**/*.md' --tab-width 4 --prose-wrap always
 
@@ -153,10 +163,15 @@ USER builder
 # prerequisites. `make`, not the script directly, because the Makefile's
 # `export CGO_ENABLED = 0` applies only to what it invokes.
 #
-# Third 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.
+# Third per-stage declaration of the gate cache-buster and its check;
+# 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 if [ -z "${CHECK_EPOCH}" ]; then \
+        echo "CHECK_EPOCH is unset; build via script/cibuild or script/docker" >&2; \
+        exit 1; \
+    fi
 RUN echo "gate test, epoch ${CHECK_EPOCH}" && make test
 
 # The version stamped into the binary: the VERSION build argument when

README.md

@@ -821,9 +821,12 @@ from binary-versus-pin to pin-versus-pin, which is what
 gate layers from cache and the build exits 0 having executed no tests and no
 lint — a green it never earned, and one this repository has produced twice.
 `CHECK_EPOCH` invalidates the gate layers on every run while leaving the pinned
-base images and the dependency layers cached. `script/lint`'s value carries the
-process id as well as the epoch, because two lint runs land inside the same
-second easily and a bare epoch would cache the second one.
+base images and the dependency layers cached. Each script's value carries the
+process id as well as the epoch, because two runs land inside the same second
+easily and a bare epoch would cache the second one. Each `Dockerfile` stage with
+gates fails when the value is empty, so a bare `docker build .` stops with
+`CHECK_EPOCH is unset; build via script/cibuild or script/docker` instead of
+serving the gates from cache.
 
 ## Build
 

TODO.md

@@ -28,6 +28,10 @@
 
 # Completed Steps
 
+- a bare `docker build .` fails with a message naming `script/cibuild` and
+  `script/docker` instead of serving the gates from cache (2026-10-04,
+  https://git.eeqj.de/sneak/sfdupes/issues/39)
+
 - `make fmt` and `make fmt-check` run prettier over all Markdown, in Docker, and
   CI checks it; all Markdown reformatted (2026-10-04,
   https://git.eeqj.de/sneak/sfdupes/issues/19)
@@ -298,18 +302,19 @@
   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
+  61.1s; `script/docker` 61.1s then 53.4s), and thirteen steps were still served
+  `CACHED` in the steady state — the lint stage's `WORKDIR /src`, 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 any linter already on `PATH` satisfied the

script/cibuild

@@ -21,14 +21,17 @@
 # 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.
+# go mod download cached; see the Dockerfile for the placement. The
+# process id goes in with the epoch so that two runs started in the
+# same second still get different values, the same form script/lint
+# uses.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
 main() {
     cd "$ROOT"
-    docker build --build-arg CHECK_EPOCH="$(date +%s)" .
+    docker build --build-arg CHECK_EPOCH="$(date +%s)-$$" .
 }
 
 main "$@"

script/docker

@@ -3,7 +3,7 @@
 # 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
+# without a fresh value Docker serves the gate layers from cache on an
 # unchanged tree and this exits 0 having run none of the lint stage's
 # gates, the markdown stage's prettier gate or the builder stage's test
 # gate. This is the set of gates a developer or reviewer runs by hand,
@@ -17,7 +17,7 @@ ROOT="$(cd "$SCRIPT_DIR/.." && pwd -P)"
 main() {
     cd "$ROOT"
     docker build \
-        --build-arg CHECK_EPOCH="$(date +%s)" \
+        --build-arg CHECK_EPOCH="$(date +%s)-$$" \
         -t "$("$SCRIPT_DIR/projectname")" \
         .
 }