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

script/cibuild

@@ -1,15 +1,19 @@
 #!/bin/sh
 # script/cibuild: run the CI build. The Dockerfile runs the checks
-# (make fmt-check, lint, test), so a successful build implies a green
-# repo. Generic: needs no adaptation. The Gitea workflow runs this on
-# push.
+# (make fmt-check, lint, test) as build steps. This script passes a new
+# CHECK_EPOCH on every run, so Docker runs those steps instead of
+# reusing cached results: a successful run means the checks passed on
+# this tree. A plain `docker build .` can reuse them and proves nothing
+# by itself. Generic: needs no adaptation. The Gitea workflow runs this
+# on push.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
 main() {
     cd "$ROOT"
-    docker build .
+    epoch="$(date +%s)$$"
+    docker build --build-arg CHECK_EPOCH="$epoch" .
 }
 
 main "$@"