Add image metadata, allow flags to be overridden, and tzdata

That last one was a major omission and would cause the daemon to hang
when run in a container.

Dockerfile

@@ -1,4 +1,16 @@
 FROM alpine:latest
+
+LABEL org.opencontainers.image.title=NXTP
+LABEL org.opencontainers.image.description="An NXTP client and server"
+LABEL org.opencontainers.image.vendor="Keith Gaughan"
+LABEL org.opencontainers.image.licenses=MIT
+LABEL org.opencontainers.image.url=https://github.com/kgaughan/nxtp
+LABEL org.opencontainers.image.source=https://github.com/kgaughan/nxtp
+LABEL org.opencontainers.image.documentation=https://kgaughan.github.io/nxtp/
+
+RUN apk --no-cache add tzdata
 COPY nxtp .
+USER nobody
 EXPOSE 12300
-ENTRYPOINT ["/nxtp", "-endpoint", ":12300"]
+ENTRYPOINT ["/nxtp"]
+CMD ["-endpoint", "0.0.0.0:12300"]