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

README.md

@@ -34,6 +34,7 @@ Image: `git.eeqj.de/sneak/baseimage`, built for `linux/amd64` and
 | pyenv           | git tag                        | `/usr/local/pyenv`             |
 | Python (latest) | `pyenv install`                | `/usr/local/pyenv/versions/*`  |
 | virtualenv      | apt, and pip in pyenv Python   | `virtualenv` on `PATH`         |
+| tzdata          | apt                            | `/usr/share/zoneinfo`          |
 
 Plus the usual odds and ends: `git`, `curl`, `wget`, `jq`, `rsync`,
 `openssh-client`, `zip`/`unzip`, `xz-utils`, `bzip2`, and the `-dev`