Use multistage build in Dockerfile to shrink image size

Co-Authored-By: betanummeric <40263343+betanummeric@users.noreply.github.com>

Dockerfile

@@ -1,13 +1,19 @@
-FROM node:8-alpine
-
+### STAGE 1: Build ###
+FROM node:8-alpine as build
+WORKDIR /srv
 COPY js /srv/js
 COPY pages /srv/pages
 COPY static /srv/static
 COPY next.config.js package.json package-lock.json server.js style.css yarn.lock /srv/
-WORKDIR /srv
 RUN yarn install
 RUN yarn build
+RUN yarn cache clean
 
+### STAGE 2: Productive Container ###
+FROM alpine
+WORKDIR /srv
+COPY --from=build /srv /srv
+RUN apk --no-cache add yarn && rm -rf /var/cache/apk/*
 EXPOSE 80
 ENV TURNIERE_API_URL=https://api.turnie.re
 CMD yarn start