Optimize Docker image

Dockerfile

@@ -1,9 +1,18 @@
-FROM node:12
-WORKDIR /app
-COPY . ./
+# Stage 1 - build app
+FROM node:12 as build-deps
+WORKDIR /usr/src/app
+COPY package.json yarn.lock ./
 RUN yarn
+COPY . ./
 RUN yarn build
+
+# Stage 2 - run
+FROM node:12-alpine
+WORKDIR /app
+COPY --from=build-deps /usr/src/app/build /app/build
+COPY --from=build-deps /usr/src/app/public /app/public
+COPY --from=build-deps /usr/src/app/node_modules /app/node_modules
 VOLUME /app/config
 VOLUME /app/content
 EXPOSE 3000
-CMD [ "yarn", "start" ]
+CMD [ "node", "build/server.js" ]