define port in environment variable

Dockerfile

@@ -1,3 +1,5 @@
+ARG PORT=3000
+ENV PORT=$PORT
 # Use the official lightweight Node.js 12 image.
 # https://hub.docker.com/_/node
 FROM node:12-alpine
@@ -11,6 +13,6 @@ COPY / ./
 RUN npm install
 # Compile to production
 RUN npm run build
-EXPOSE 3000
+EXPOSE ${PORT}
 # Run the web service on container startup.
 CMD [ "npm", "start" ]