Add tzdata (closes #4)

Install tzdata from apt so the time zone database is present at run
time, and check a zone lookup in the smoke test.

Model: deepseek-v4-pro

bin/test

@@ -23,6 +23,7 @@ docker run --rm "$img" bash -ec '
     prettier --version
     shellcheck --version
     govulncheck -version
+    test -e /usr/share/zoneinfo/America/New_York
 '
 
 echo "== $img: runit service lifecycle"