Add ENTRYPOINT for convenience

Now we can use following instructions to run.
docker run --net=host --cap-add=NET_ADMIN wangyucn/udp2raw-tunnel  -c -r44.55.66.77:9000 -l 127.0.0.1:6000 --raw-mode faketcp --lower-level auto -a -k xxxxxxxx

Dockerfile

@@ -10,3 +10,4 @@ RUN apk add --no-cache git  build-base linux-headers && \
 FROM alpine:3.6
 RUN apk add --no-cache libstdc++ iptables
 COPY --from=builder /udp2raw-tunnel/udp2raw_dynamic /bin/
+ENTRYPOINT [ "/bin/udp2raw_dynamic" ]