reduce container image size from 845MB to 98MB (#32)

Thank you!

Dockerfile

-FROM ubuntu:latest
-
-EXPOSE 3478/tcp 3478/udp
-
-USER root
+FROM ubuntu:18.04 as build
 
 RUN set -ex && \
     apt-get update && \
@@ -17,8 +13,20 @@ RUN set -ex && \
 
 RUN cd /opt && git clone https://github.com/jselbie/stunserver.git && cd stunserver && make
 
+FROM ubuntu:18.04
+
+EXPOSE 3478/tcp 3478/udp
+
+RUN apt update && apt install libssl1.1 && apt-get clean
+
+RUN mkdir /opt/stunserver
+COPY --from=build /opt/stunserver/stunclient /opt/stunserver/stunclient
+COPY --from=build /opt/stunserver/stunserver /opt/stunserver/stunserver
+
 WORKDIR /opt/stunserver
 
 HEALTHCHECK CMD /opt/stunserver/stunclient localhost
 
 ENTRYPOINT ["/opt/stunserver/stunserver"]
+
+