Edited Dockerfile of Docker(Nginx) deployment doc (#4561)

* Edited Dockerfile of Docker(Nginx) deployment doc

Edited `Dockerfile` section of https://cli.vuejs.org/guide/deployment.html#docker-nginx

As shown in [Vue v2 cookbook](https://vuejs.org/v2/cookbook/dockerize-vuejs-app.html) copying `package*.json` initially and running `npm install` in a separate step allows caching and reduces time elapsed during Docker build. Also the difference between the two docs will be reduced and be less confusing to those who end up with both of them.

* node version to latest

docs/guide/deployment.md

@@ -395,14 +395,16 @@ Deploy your application using nginx inside of a docker container.
 2. Create a `Dockerfile` file in the root of your project.
 
     ```Dockerfile
-    FROM node:10
-    COPY ./ /app
+    FROM node:latest as build-stage
     WORKDIR /app
-    RUN npm install && npm run build
+    COPY package*.json ./
+    RUN npm install
+    COPY ./ .
+    RUN npm run build
 
-    FROM nginx
+    FROM nginx as production-stage
     RUN mkdir /app
-    COPY --from=0 /app/dist /app
+    COPY --from=build-stage /app/dist /app
     COPY nginx.conf /etc/nginx/nginx.conf
     ```