Run the checks on every script/cibuild and script/docker build (closes #101)

Both scripts pass a new CHECK_EPOCH, which the Dockerfile's make
fmt-check, make lint and make test steps name in their commands. On an
unchanged tree Docker used to serve those steps from its build cache, so
a run could pass without checking anything. The script/bootstrap steps
stay cached. A plain docker build . still works, as upaas builds the
image that way: it leaves CHECK_EPOCH empty and reuses the check steps
only for an identical build context.

Model: opus-5-5

TODO.md

@@ -29,6 +29,15 @@ P2: security: referer blacklist
 
 # Completed Steps
 
+- 2026-10-03 every `script/cibuild` and `script/docker` run executes the checks
+  (closes #101): the `Dockerfile` declares `CHECK_EPOCH` above `make fmt-check`
+  and `make lint` in the lint stage and above `make test` in the build stage,
+  and each of those steps names it in its command; both scripts pass a new value
+  on every run, so Docker runs the checks instead of reusing cached results,
+  while the `script/bootstrap` steps stay cached; a plain `docker build .` still
+  works, leaves it empty, and reuses the check steps only for an identical build
+  context; the `script/cibuild` comment and `README.md` no longer say that any
+  successful build implies a green repo.
 - 2026-09-29 share concurrent misses (closes #65): requests that miss the same
   variant at once (the same cache key, so quality and fit included) share one
   upstream fetch or cached source read and one transcode through