Run the checks again on every script/cibuild (closes #54)

On an unchanged tree docker served every check step of the Dockerfile
from its build cache, so a second script/cibuild ran no lint, tests or
build and still succeeded.

script/cibuild now passes the current time as the CHECK_EPOCH build
argument. The lint and build stages each declare it after their module
download and before `COPY . .`. A build argument whose value changes
makes every RUN step after its declaration miss the cache, so the
checks run on each build while the base images, the apk install and the
module downloads stay cached.

Model: opus-5-5

script/cibuild

@@ -4,13 +4,17 @@
 # The Gitea workflow runs this on push. The memlock ulimit lets the tests
 # that lock large secrets in memory (memguard mlocks them) run; under the
 # lower limit of a plain `docker build .` they are skipped.
+# A cached build checks nothing: a new CHECK_EPOCH on every run makes the
+# Dockerfile's check steps run again on an unchanged tree, while its base
+# images and module downloads stay cached.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
 main() {
     cd "$ROOT"
-    docker build --ulimit memlock=-1:-1 .
+    docker build --ulimit memlock=-1:-1 \
+        --build-arg CHECK_EPOCH="$(date +%s)" .
 }
 
 main "$@"