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.

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 "$@"