diff --git a/Dockerfile b/Dockerfile
index df5ea97..8e852de 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -6,6 +6,11 @@ WORKDIR /src
 COPY go.mod go.sum ./
 RUN go mod download
 
+# script/cibuild sets CHECK_EPOCH to the current time, so the RUN steps
+# below run again on each build, an unchanged tree included, while the
+# steps above stay cached. ARG is per stage: the build stage declares it too.
+ARG CHECK_EPOCH
+
 COPY . .
 
 RUN make fmt-check
@@ -25,6 +30,9 @@ WORKDIR /build
 COPY go.mod go.sum ./
 RUN go mod download
 
+# As in the lint stage: the RUN steps below run again on each script/cibuild.
+ARG CHECK_EPOCH
+
 COPY . .
 
 RUN make test
diff --git a/README.md b/README.md
index 29d497d..f89b5a6 100644
--- a/README.md
+++ b/README.md
@@ -523,7 +523,8 @@ them. We provide:
 - `script/docker` — build the Docker image tagged with the project name
 - `script/cibuild` — CI entrypoint: `docker build --ulimit
   memlock=-1:-1 .` (memguard needs mlock; the Dockerfile runs the
-  checks)
+  checks), with a new `CHECK_EPOCH` build argument on every run so the
+  checks run again on an unchanged tree
 - `script/precommit` — pre-commit checks: `go mod tidy` verification,
   then `script/check`
 - `script/install-precommit` — install the git pre-commit hook that
diff --git a/TODO.md b/TODO.md
index f04fe2f..d89b291 100644
--- a/TODO.md
+++ b/TODO.md
@@ -25,6 +25,14 @@ Bring the repo into policy compliance in one commit:
 
 # Completed Steps
 
+- 2026-10-04: `script/cibuild` runs the checks again on an unchanged
+  tree (https://git.eeqj.de/sneak/secret/issues/54). It passes the
+  current time as the `CHECK_EPOCH` build argument, which both the lint
+  and the build stage of the `Dockerfile` declare after their module
+  download, so the `RUN` steps below the argument run again on each
+  build while the base images and module downloads stay cached. Before,
+  a second run on the same tree took every check from the build cache
+  and reported success having run nothing.
 - 2026-10-04: A failed unlocker add no longer leaves a partial unlocker
   directory (https://git.eeqj.de/sneak/secret/issues/48).
   `secret unlocker add pgp` resolves the GPG key's fingerprint once, for
diff --git a/script/cibuild b/script/cibuild
index 3316194..58bf15f 100755
--- a/script/cibuild
+++ b/script/cibuild
@@ -4,13 +4,17 @@
 # The Gitea workflow runs this on push. The memlock ulimit lets the tests
 # that lock large secrets in memory (memguard mlocks them) run; under the
 # lower limit of a plain `docker build .` they are skipped.
+# A cached build checks nothing: a new CHECK_EPOCH on every run makes the
+# Dockerfile's check steps run again on an unchanged tree, while its base
+# images and module downloads stay cached.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
 main() {
     cd "$ROOT"
-    docker build --ulimit memlock=-1:-1 .
+    docker build --ulimit memlock=-1:-1 \
+        --build-arg CHECK_EPOCH="$(date +%s)" .
 }
 
 main "$@"