Bust the Docker check-layer cache with a per-invocation CHECK_EPOCH (closes #26)

script/cibuild was a plain `docker build .`, and the Dockerfile does
`COPY . .` followed by `RUN make check`. Docker invalidates a COPY layer
only when the copied content changes, so on an unchanged tree the check
layer was served from cache, the suite never ran, and the build still
exited 0. Measured here: run 1 took 18.5s and ran the suite; run 2 on a
byte-identical tree took 0.286s with `RUN make check` CACHED.

script/cibuild and script/docker now assign a per-invocation nonce on its
own line and pass it as --build-arg CHECK_EPOCH. The Dockerfile declares
ARG CHECK_EPOCH, guards it with `[ -n "$CHECK_EPOCH" ] || exit 1`, and
expands it into the check command. Post-fix, two consecutive runs both
execute make check (17.4s / 8.1s) with `RUN script/bootstrap` still
CACHED, so dependency layers are untouched and the build ceiling is not
at risk.

The guard is what makes a bare `docker build .` — the command
REPO_POLICIES named verbatim — fail closed rather than reuse the empty
and therefore stable cache key; verified failing in 0.455s. Holding the
epoch constant restores the false green (run 2 fully CACHED), which pins
the varying value as the operative mechanism rather than a coincidence.

Because the guard references $CHECK_EPOCH it is itself value-keyed:
BuildKit renders the epoch into that layer's description and re-runs the
layer when the value changes. Each stage therefore has two independent
invalidation points, the guard and the expansion, and the guard always
precedes the check RUN. Both are kept and the prose now records this;
the expansion remains defence in depth and is what puts the epoch in the
build log.

The false guarantee was org-canonical text in more than one document, so
it is corrected everywhere it appeared rather than only where the issue
first found it. REPO_POLICIES.md carried it in two places, and its Go
multistage template had check steps in two stages; ARG is stage-scoped,
so both stages get the treatment or the fleet inherits the half-fixed
shape. CODE_STYLEGUIDE_GO.md restated the guarantee for the bare command
this change makes fail closed. NEW_REPO_CHECKLIST.md specified the
pre-fix script/cibuild verbatim, so every new repo would have been born
with the false green, and EXISTING_REPO_CHECKLIST.md ended on a
`docker build` acceptance item that the guard makes unsatisfiable by
design — an agent working that checklist would have been led to delete
the guard to tick the last box. Both checklists' Dockerfile criteria were
also satisfiable by a Dockerfile whose check layers are still frozen, and
now require the ARG and guard in every check-running stage.

REPO_POLICIES.md's own Dockerfile criterion carried that same incomplete
form; it is tightened by cross-reference to the CHECK_EPOCH rule rather
than by duplicating the canonical block. The Go template's Key points
gain a caveat that the cache-bust turns the `COPY --from=lint` no-op into
a content-cache hit, so a repo using a file-dependency trick for stage
ordering must re-prove that ordering on a warm cache after adopting it.
That was re-proved in another repo in the org which uses the trick with a
marker file, where the ordering held; the caveat states explicitly that
it was not verified here, this repo being single-stage with no lint stage
to order against.

Dockerfile

@@ -12,4 +12,17 @@ COPY package.json yarn.lock ./
 RUN script/bootstrap
 
 COPY . .
-RUN make check
+
+# CHECK_EPOCH is a per-invocation nonce supplied by script/cibuild and
+# script/docker. Without it an unchanged tree serves this layer from
+# cache and the build reports a green it never ran. ARG is stage-scoped,
+# so it must be redeclared in every stage that runs checks. The guard
+# makes a bare `docker build .` fail loudly instead of silently reusing
+# the empty (and therefore stable) cache key. Expand the value into the
+# command so the cache miss does not depend on BuildKit's handling of an
+# unreferenced ARG. Both the guard and the check RUN reference the value,
+# so both are value-keyed: there are two independent invalidation points
+# here, not one. Keep both.
+ARG CHECK_EPOCH
+RUN [ -n "$CHECK_EPOCH" ] || exit 1
+RUN echo "check epoch: ${CHECK_EPOCH}" && make check

README.md

@@ -123,9 +123,13 @@ alpine. We provide:
 - `script/check` — run all checks: `test`, `lint`, `fmt-check` (our own
   extension)
 - `script/docker` — build the Docker image, tagged via `script/projectname`
-  (byte-identical across repos)
+  (byte-identical across repos); passes the same `CHECK_EPOCH` nonce as
-- `script/cibuild` — cd to the repo root and `docker build .` (what CI runs; the
+  `script/cibuild`
-  image build runs `script/check`)
+- `script/cibuild` — cd to the repo root, assign `epoch="$(date +%s%N)$$"`, then
+  `docker build --build-arg CHECK_EPOCH="$epoch" .` (what CI runs; the image
+  build runs `script/check`, and the per-invocation `CHECK_EPOCH` nonce is what
+  stops Docker serving that check from cache on an unchanged tree — a bare
+  `docker build .` fails closed on purpose)
 - `script/precommit` — run by the git pre-commit hook (our own extension); calls
   `script/check`
 - `script/install-precommit` — installs the git pre-commit hook (our own

TODO.md

@@ -21,9 +21,15 @@ fmt-check, and commit.
 
 # Completed Steps
 
-- 2026-08-10: Added `-count=1` to both `go test` invocations in the canonical Go
+- 2026-08-09: Fixed the false green in the canonical CI gate: `script/cibuild`
-  `make test` example in `REPO_POLICIES.md`, so the target cannot report a
+  and `script/docker` now pass a per-invocation `CHECK_EPOCH` nonce, and the
-  cached pass it did not earn.
+  `Dockerfile` (plus the Go multistage template in REPO_POLICIES.md, in both its
+  lint and builder stages) declares `ARG CHECK_EPOCH` with a guard that makes a
+  bare `docker build .` fail closed. Corrected the org-canonical text that
+  asserted a successful build implies all checks pass, across every document
+  carrying it: `REPO_POLICIES.md`, both repo checklists (which still told agents
+  to write the pre-fix `script/cibuild` and ended on an acceptance item the
+  guard makes unsatisfiable), and the Go styleguide.
 - 2026-08-07: Set the canonical `.golangci.yml` to the org-standard v2-schema
   config already deployed byte-identical across the org's Go repos (settings
   under `linters.settings` so thresholds like lll/funlen/cyclop/dupl actually

prompts/CODE_STYLEGUIDE_GO.md

@@ -1,6 +1,6 @@
 ---
 title: Code Styleguide — Go
-last_modified: 2026-03-18
+last_modified: 2026-08-09
 ---
 
 1.  Try to hard wrap long lines at 77 characters or less.
@@ -101,9 +101,16 @@ last_modified: 2026-03-18
     `golangci-lint`.
 
 1.  Write a `Dockerfile` for every repo, even if it only runs the tests and
-    linting. `docker build .` should always make sure that the code is in an
+    linting. `script/cibuild` and `script/docker` should always make sure that
-    able-to-be-compiled state, linted, and any tests run. The Docker build
+    the code is in an able-to-be-compiled state, linted, and any tests run, and
-    should fail if linting doesn't pass.
+    the build should fail if linting doesn't pass. That guarantee holds only
+    because those scripts pass a per-invocation `CHECK_EPOCH` build arg that
+    busts the check layers out of the Docker cache; without it an unchanged tree
+    serves those layers from cache and the build reports a green it never ran. A
+    bare `docker build .` fails closed by design, on the `[ -n "$CHECK_EPOCH" ]`
+    guard — always go through `script/cibuild` or `script/docker`. See
+    [Repository Policies](https://git.eeqj.de/sneak/prompts/raw/branch/main/prompts/REPO_POLICIES.md)
+    for the canonical form.
 
 1.  Every repo must have a `Makefile`. See
     [Repository Policies](https://git.eeqj.de/sneak/prompts/raw/branch/main/prompts/REPO_POLICIES.md)

prompts/EXISTING_REPO_CHECKLIST.md

@@ -1,6 +1,6 @@
 ---
 title: Existing Repo Checklist
-last_modified: 2026-07-06
+last_modified: 2026-08-09
 ---
 
 Use this checklist when beginning work in a repo that may not yet conform to our
@@ -29,10 +29,15 @@ with your task.
       if missing
 - [ ] `.editorconfig` exists — fetch from
       `https://git.eeqj.de/sneak/prompts/raw/branch/main/.editorconfig`
-- [ ] `Dockerfile` and `.dockerignore` exist; Dockerfile runs `make check` as a
+- [ ] `Dockerfile` and `.dockerignore` exist (fetch `.dockerignore` from
-      build step — fetch `.dockerignore` from
+      `https://git.eeqj.de/sneak/prompts/raw/branch/main/.dockerignore`);
-      `https://git.eeqj.de/sneak/prompts/raw/branch/main/.dockerignore`
+      Dockerfile runs `make check` as a build step, and every stage containing a
-- [ ] Gitea Actions workflow in `.gitea/workflows/` runs `docker build .` on
+      check-running `RUN` declares `ARG CHECK_EPOCH` with the
+      `RUN [ -n "$CHECK_EPOCH" ] || exit 1` guard immediately below it — see the
+      `CHECK_EPOCH` rule in `REPO_POLICIES.md`. Without them the check layer is
+      served from cache on an unchanged tree and the build reports a green it
+      never ran.
+- [ ] Gitea Actions workflow in `.gitea/workflows/` runs `script/cibuild` on
       push — reference
       `https://git.eeqj.de/sneak/prompts/raw/branch/main/.gitea/workflows/check.yml`
 - [ ] Language-specific config:
@@ -104,5 +109,6 @@ with your task.
 # Final
 
 - [ ] `make check` passes
-- [ ] `docker build` succeeds
+- [ ] `script/cibuild` succeeds (a bare `docker build .` fails closed by design,
+      on the `CHECK_EPOCH` guard)
 - [ ] Commit and merge fixes before starting your actual task

prompts/NEW_REPO_CHECKLIST.md

@@ -1,6 +1,6 @@
 ---
 title: New Repo Checklist
-last_modified: 2026-07-06
+last_modified: 2026-08-09
 ---
 
 Use this checklist when creating a new repository from scratch. Follow the steps
@@ -52,7 +52,12 @@ Template files can be fetched from:
       `https://git.eeqj.de/sneak/prompts/raw/branch/main/prompts/REPO_POLICIES.md`
 - [ ] `Dockerfile` and `.dockerignore` — fetch `.dockerignore` from
       `https://git.eeqj.de/sneak/prompts/raw/branch/main/.dockerignore`
-    - All Dockerfiles must run `make check` as a build step
+    - All Dockerfiles must run `make check` as a build step, and every stage
+      containing a check-running `RUN` must declare `ARG CHECK_EPOCH` with the
+      `RUN [ -n "$CHECK_EPOCH" ] || exit 1` guard immediately below it — see the
+      `CHECK_EPOCH` rule in `REPO_POLICIES.md`. Without them the check layer is
+      served from cache on an unchanged tree and the build reports a green it
+      never ran.
     - Server: also builds and runs the application
     - Non-server: brings up dev environment and runs `make check`
     - Image pinned by sha256 hash with version/date comment
@@ -90,8 +95,14 @@ are thin shims calling them. Model scripts:
 - [ ] `script/projectname` — outputs the project name (used by `script/docker`
       for the image tag)
 - [ ] `script/docker` / `make docker` — builds Docker image, tagged via
-      `script/projectname` (byte-identical across repos)
+      `script/projectname` (byte-identical across repos); assigns
-- [ ] `script/cibuild` — cd to repo root, `docker build .` (what CI runs)
+      `epoch="$(date +%s%N)$$"` on its own line and passes
+      `--build-arg CHECK_EPOCH="$epoch"`
+- [ ] `script/cibuild` — cd to repo root, assign `epoch="$(date +%s%N)$$"` on
+      its own line, then run `docker build --build-arg CHECK_EPOCH="$epoch" .`
+      (what CI runs). The build arg is mandatory: see the `CHECK_EPOCH` rule in
+      `REPO_POLICIES.md` for why each element is load-bearing. A bare
+      `docker build .` fails closed by design.
 - [ ] `script/precommit` — called by the pre-commit hook; runs `script/check`
 - [ ] `script/install-precommit` — installs the pre-commit hook that runs
       `script/precommit`

prompts/REPO_POLICIES.md

@@ -1,6 +1,6 @@
 ---
 title: Repository Policies
-last_modified: 2026-08-10
+last_modified: 2026-08-09
 ---
 
 This document covers repository structure, tooling, and workflow standards. Code
@@ -60,17 +60,19 @@ style conventions are in separate documents:
   prerequisite since nvm requires bash. yarn is then pinned via
   `corepack prepare yarn@<version> --activate`. Never install "latest" or "lts";
   always exact versions. `script/cibuild` runs the CI build: it changes to the
-  repo root and runs `docker build .`; the Gitea workflow calls it. Four further
+  repo root and runs `docker build --build-arg CHECK_EPOCH="$epoch" .`, where
-  scripts are our own extensions to the standard: `script/check` runs
+  `epoch` is a per-invocation nonce (see the `CHECK_EPOCH` rule below); the
-  `script/test`, `script/lint`, and `script/fmt-check`; `script/precommit` is
+  Gitea workflow calls it. Four further scripts are our own extensions to the
-  what the git pre-commit hook runs, and it calls `script/check`;
+  standard: `script/check` runs `script/test`, `script/lint`, and
-  `script/install-precommit` installs the git pre-commit hook (the `make hooks`
+  `script/fmt-check`; `script/precommit` is what the git pre-commit hook runs,
-  target shims to it); and `script/projectname` (literally that filename) simply
+  and it calls `script/check`; `script/install-precommit` installs the git
-  outputs the project's name. Scripts that need the name call
+  pre-commit hook (the `make hooks` target shims to it); and
-  `script/projectname` — e.g. `script/docker` assembles its image tag from it —
+  `script/projectname` (literally that filename) simply outputs the project's
-  so those scripts stay byte-identical across all repos. Repo-type-specific
+  name. Scripts that need the name call `script/projectname` — e.g.
-  pre-commit extras (e.g. `go mod tidy` verification in Go repos) belong in
+  `script/docker` assembles its image tag from it — so those scripts stay
-  `script/precommit`, not in the hook itself. Model scripts are at
+  byte-identical across all repos. Repo-type-specific pre-commit extras (e.g.
+  `go mod tidy` verification in Go repos) belong in `script/precommit`, not in
+  the hook itself. Model scripts are at
   `https://git.eeqj.de/sneak/prompts/raw/branch/main/script/<name>`. The README
   must document the provided scripts in an **Entrypoints** section (see the
   README requirements below).
@@ -90,14 +92,70 @@ style conventions are in separate documents:
   reading the Makefile.
 
 - Every repo should have a `Dockerfile`. All Dockerfiles must run `make check`
-  as a build step so the build fails if the branch is not green. For non-server
+  as a build step so the build fails if the branch is not green — which requires
-  repos, the Dockerfile should bring up a development environment and run
+  `ARG CHECK_EPOCH` and its guard in every stage containing a check-running
-  `make check`. For server repos, `make check` should run as an early build
+  `RUN`, per the `CHECK_EPOCH` rule below. Without them a Dockerfile satisfies
-  stage before the final image is assembled. Dockerfiles install development
+  this criterion while its check layers are served from cache, so the build
-  prerequisites by running `script/bootstrap` rather than duplicating installs
+  cannot fail on a branch that is not green. For non-server repos, the
-  inline; COPY `script/` and the dependency manifests (`package.json` +
+  Dockerfile should bring up a development environment and run `make check`. For
-  `yarn.lock`, `go.mod` + `go.sum`, etc.) before running it so the bootstrap
+  server repos, `make check` should run as an early build stage before the final
-  layer stays cached until dependencies change.
+  image is assembled. Dockerfiles install development prerequisites by running
+  `script/bootstrap` rather than duplicating installs inline; COPY `script/` and
+  the dependency manifests (`package.json` + `yarn.lock`, `go.mod` + `go.sum`,
+  etc.) before running it so the bootstrap layer stays cached until dependencies
+  change.
+
+- **Every check-running `RUN` must be cache-busted with `CHECK_EPOCH`.** Docker
+  invalidates a `COPY` layer only when the copied content changes, so on an
+  unchanged tree the `RUN make check` layer is served from cache, the suite
+  never runs, and the build still exits 0. A sub-second `docker build` reporting
+  success is a cache hit, not a result. The canonical form, in **every** stage
+  containing a check-running `RUN`:
+
+    ```dockerfile
+    ARG CHECK_EPOCH
+    RUN [ -n "$CHECK_EPOCH" ] || exit 1
+    RUN echo "check epoch: ${CHECK_EPOCH}" && make check
+    ```
+
+    and in both `script/cibuild` and `script/docker`:
+
+    ```sh
+    epoch="$(date +%s%N)$$"
+    docker build --build-arg CHECK_EPOCH="$epoch" .
+    ```
+
+    All four elements are load-bearing; none is optional, and each guards a
+    failure mode that otherwise fails green:
+    - `ARG` is stage-scoped, so a single declaration leaves the other check
+      stages frozen while the fix reviews as complete. Declare it in every stage
+      that runs checks, immediately above the first such `RUN`.
+    - Expand the value into the command. This makes the cache miss contractual
+      rather than dependent on BuildKit's handling of an unreferenced `ARG`, and
+      it puts the epoch in the build log. The guard is itself value-keyed, for
+      the same reason: it references `$CHECK_EPOCH`, so BuildKit renders the
+      epoch into that layer's description (rendered as
+      `RUN [ -n "<epoch>" ] || exit 1`) and re-runs it whenever the value
+      changes. Each stage therefore has two independent invalidation points, and
+      the guard always precedes the check `RUN`. Keep both: the expansion is
+      defence in depth, and it is what makes the epoch visible in the build
+      output.
+    - The `[ -n ... ]` guard is required: an unset `ARG` is empty, and empty is
+      a stable cache key, so without it a bare `docker build .` still produces
+      the false green. Failed steps are never cached, so the guard fails on
+      every such invocation, loudly. A bare `docker build .` failing is by
+      design.
+    - Assign `epoch=` on its own line, never inline in the `--build-arg`
+      argument: a failing command substitution inside an argument does not trip
+      `set -e`, so the inline form silently degrades to an empty constant. The
+      `$$` suffix is required because busybox `date` drops `%N` and exits 0, so
+      on an alpine host the epoch would degrade to second granularity and
+      concurrent invocations would collide.
+
+    This invalidates the check layers and everything after them while leaving
+    `go mod download`, `script/bootstrap`, and the pinned toolchain install
+    cached, so it does not push against the five-minute Docker build ceiling.
+    Blanket `--no-cache` also works but is wasteful and can blow that ceiling.
 
 - **Dockerfiles must use a separate lint stage for fail-fast feedback.** Go
   repos use a multistage build where linting runs in an independent stage based
@@ -119,7 +177,9 @@ style conventions are in separate documents:
     COPY go.mod go.sum ./
     RUN go mod download
     COPY . .
-    RUN make fmt-check
+    ARG CHECK_EPOCH
+    RUN [ -n "$CHECK_EPOCH" ] || exit 1
+    RUN echo "check epoch: ${CHECK_EPOCH}" && make fmt-check
     RUN make lint
 
     # Build stage
@@ -133,7 +193,9 @@ style conventions are in separate documents:
     COPY go.mod go.sum ./
     RUN go mod download
     COPY . .
-    RUN make test
+    ARG CHECK_EPOCH
+    RUN [ -n "$CHECK_EPOCH" ] || exit 1
+    RUN echo "check epoch: ${CHECK_EPOCH}" && make test
 
     ARG VERSION=dev
     RUN CGO_ENABLED=0 go build -trimpath \
@@ -154,6 +216,16 @@ style conventions are in separate documents:
       a stage dependency. BuildKit runs stages in parallel by default; without
       this line, the build stage would not wait for lint to finish and a lint
       failure might not fail the overall build.
+    - **Re-prove that ordering on a warm cache after adopting `CHECK_EPOCH`.**
+      The cache-bust turns this no-op `COPY` into a content-cache hit, so an
+      ordering guarantee established on a cold cache does not automatically
+      carry over; it has to be re-checked warm. This was re-proved in another
+      repo in the org that uses the same file-dependency trick (there with a
+      marker file in place of `go.sum`), and the ordering held. It has **not**
+      been verified in this repo, which is single-stage and has no lint stage to
+      order against. Any repo relying on a file-dependency trick for stage
+      ordering should re-check it warm after adopting the bust rather than
+      assuming this result transfers.
     - If the project uses `//go:embed` directives that reference build artifacts
       (e.g. a web frontend compiled in a separate stage), the lint stage must
       create placeholder files so the embed directives resolve. Example:
@@ -165,11 +237,28 @@ style conventions are in separate documents:
     - The build stage runs `make test` after compilation setup. Tests run in the
       build stage, not the lint stage, because they may require compiled
       artifacts or heavier dependencies.
+    - `ARG CHECK_EPOCH` appears in **both** stages, because `ARG` is
+      stage-scoped: declaring it only in the lint stage leaves `make test`
+      frozen at the last cached result. In each stage the guard sits immediately
+      below the `ARG` so a bare `docker build .` fails instead of reusing the
+      empty cache key, and the value is expanded into the first check `RUN` so
+      the cache miss does not rely on BuildKit's unreferenced-`ARG` handling.
+      Both of those lines reference `$CHECK_EPOCH`, so both are value-keyed:
+      each stage is invalidated at two independent points. The later `RUN`s in
+      the same stage need no expansion of their own: they are already
+      invalidated by their busted parent layer.
 
 - Every repo should have a Gitea Actions workflow (`.gitea/workflows/`) that
-  runs `script/cibuild` (which runs `docker build .`) on push. Since the
+  runs `script/cibuild` (which runs
-  Dockerfile already runs `make check`, a successful build implies all checks
+  `docker build --build-arg CHECK_EPOCH="$epoch" .`) on push. The Dockerfile
-  pass.
+  runs `make check`, so a successful build implies all checks pass — but that
+  implication holds **only** because of the `CHECK_EPOCH` cache-bust described
+  above. Without it, an unchanged tree serves the check layer from cache and the
+  build reports a green it never earned. A bare `docker build .` fails closed by
+  design, on the `[ -n "$CHECK_EPOCH" ]` guard; always go through
+  `script/cibuild` or `script/docker`. Never accept a `script/cibuild` pass as
+  evidence without confirming it ran: a sub-second wall time, or `CACHED` on the
+  check layer, means nothing was executed.
 
 - Use platform-standard formatters: `black` for Python, `prettier` for
   JS/CSS/Markdown/HTML, `go fmt` for Go. Always use default configuration with
@@ -209,16 +298,11 @@ style conventions are in separate documents:
 
     ```makefile
     test:
-    	@go test -count=1 -timeout 30s -race -cover ./... || \
+    	@go test -timeout 30s -race -cover ./... || \
     		{ echo "--- Rerunning with -v for details ---"; \
-    		  go test -count=1 -timeout 30s -race -v ./...; exit 1; }
+    		  go test -timeout 30s -race -v ./...; exit 1; }
     ```
 
-    `-count=1` is required on both invocations: it defeats Go's test _result_
-    cache, so the target cannot report a pass it did not earn, and the rerun
-    reproduces a failure instead of replaying it. It leaves the build cache
-    alone, so it costs the runtime of the suite and no recompilation.
-
     Python example:
 
     ```makefile

script/cibuild

@@ -1,13 +1,20 @@
 #!/bin/sh
-# script/cibuild: run the CI build. The Dockerfile runs script/check, so
+# script/cibuild: run the CI build. The Dockerfile runs script/check, but
-# a successful build implies all checks pass.
+# that only proves anything because CHECK_EPOCH is a fresh nonce on every
+# invocation: without it Docker serves the check layer from cache on an
+# unchanged tree and the build exits 0 without running the suite.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
 main() {
     cd "$ROOT"
-    docker build .
+    # Assign on its own line: a failing command substitution inside an
+    # argument does not trip `set -e`, which would silently degrade the
+    # nonce to an empty constant. `$$` is required because busybox `date`
+    # drops %N without erroring.
+    epoch="$(date +%s%N)$$"
+    docker build --build-arg CHECK_EPOCH="$epoch" .
 }
 
 main "$@"

script/docker

@@ -1,6 +1,9 @@
 #!/bin/sh
 # script/docker: build the Docker image tagged with the project name.
-# Identical in all repos; the tag comes from script/projectname.
+# Identical in all repos; the tag comes from script/projectname. The
+# Dockerfile's checks only actually run because CHECK_EPOCH is a fresh
+# nonce on every invocation; without it a warm cache turns this into a
+# green that proves nothing.
 set -eu
 
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"
@@ -8,7 +11,13 @@ ROOT="$(cd "$SCRIPT_DIR/.." && pwd -P)"
 
 main() {
     cd "$ROOT"
-    docker build -t "$("$SCRIPT_DIR/projectname")" .
+    # Assign on its own line: a failing command substitution inside an
+    # argument does not trip `set -e`, which would silently degrade the
+    # nonce to an empty constant. `$$` is required because busybox `date`
+    # drops %N without erroring.
+    epoch="$(date +%s%N)$$"
+    docker build --build-arg CHECK_EPOCH="$epoch" \
+        -t "$("$SCRIPT_DIR/projectname")" .
 }
 
 main "$@"