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
sneak