Bust the Docker layer cache for the gate steps (closes #32)

script/cibuild and script/docker were bare docker build invocations
with no cache control, and the Dockerfile copies the tree before
running its gates. On an unchanged tree Docker served those layers
from cache, so the gates never executed and the build still exited 0.
A merge commit here has a tree byte-identical to the branch head it
merges, so every merge CI run was almost certainly a full cache hit,
and PR #31's reviewer caught make docker returning success as a
17-layer cache hit that proved nothing.

Declare ARG CHECK_EPOCH in both stages and have the scripts pass
--build-arg CHECK_EPOCH="$(date +%s)". ARG is scoped per stage and
this Dockerfile has three gates across two of them (make fmt-check and
make lint in the lint stage, make check in the build stage), so one
declaration would have left a stage silently cacheable. BuildKit
hashes the expanded command rather than the declaration, so each gate
RUN echoes the epoch: an unreferenced ARG invalidates nothing, and the
echo doubles as evidence in the build log that the layer really ran.

Both declarations sit below the dependency layers, so the pinned base
images, go mod download, apk add and the source copies keep their
cache and only the gates go cold. The build-stage declaration sits
after USER, so the drop to the unprivileged builder user still happens
before make check and the chmod(0) permission tests stay real.

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