Fix NPM5 build dependency issue

Author: AlaricChan

Dockerfile

@@ -9,9 +9,10 @@ COPY . /RackHD/on-dhcp-proxy/
 WORKDIR /RackHD/on-dhcp-proxy
 
 RUN mkdir -p ./node_modules \
+  && npm install --ignore-scripts --production \
+  && rm -r ./node_modules/on-core ./node_modules/di \
   && ln -s /RackHD/on-core ./node_modules/on-core \
-  && ln -s /RackHD/on-core/node_modules/di ./node_modules/di \
-  && npm install --ignore-scripts --production
+  && ln -s /RackHD/on-core/node_modules/di ./node_modules/di
 
 EXPOSE 68/udp 4011
 CMD [ "node", "/RackHD/on-dhcp-proxy/index.js" ]