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

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 "$@"