build using package.json only

Dockerfile

@@ -1,10 +1,11 @@
 FROM node:8-onbuild AS build
-COPY . /app
+COPY package.json /app/package.json
 WORKDIR /app
 RUN npm install
 
 FROM node:8-alpine
 RUN apk add --update openssh-client
-COPY --from=build /app /app
+COPY . /app
+COPY --from=build /app/node_modules /app/node_modules
 WORKDIR /app
 CMD node app.js -p 3000