re-add tzdata, removed from pelican dependencies

unfortunately this is not a windows-only dependency, but required if
using timezones as we do, otherwise results in errors:

> No time zone found with key Europe/Zurich

caused by: https://gitlab.alpinelinux.org/alpine/aports/-/merge_requests/58013

Dockerfile

@@ -1,7 +1,7 @@
 FROM alpine:3.19
 WORKDIR /srv
 RUN echo "@testing https://dl-cdn.alpinelinux.org/alpine/edge/testing" >> /etc/apk/repositories
-RUN apk add --no-cache py3-pelican@testing make
+RUN apk add --no-cache py3-pelican@testing make tzdata
 COPY . /srv/
 RUN make publish