Add tzdata in final docker image

Currently, docker container only shows timestamps in UTC timezone. Installing `tzdata` will enable users to supply custom timezone.

```
docker container run -v /path/to/myFolder:/data --rm -it -e TZ=America/Los_Angeles dominikbraun/timetrace
```

Dockerfile

@@ -16,6 +16,8 @@ RUN curl -LO https://github.com/dominikbraun/timetrace/releases/download/${VERSI
 # The final stage. This is the image that will be distrubuted.
 FROM alpine:3.11.5 AS final
 
+RUN apk add -U --no-cache tzdata
+
 LABEL org.label-schema.schema-version="1.0"
 LABEL org.label-schema.name="timetrace"
 LABEL org.label-schema.description="A simple CLI for tracking your working time."
@@ -33,4 +35,4 @@ RUN mkdir /etc/timetrace && \
 
 RUN mkdir /data
 
-ENTRYPOINT ["/bin/timetrace"]
+ENTRYPOINT ["/bin/timetrace"]