build: switch to multi-stage docker build to reduce size

Dockerfile

@@ -1,4 +1,4 @@
-FROM node:12.18-alpine
+FROM node:12.18-alpine AS BUILD_IMAGE
 
 COPY . /app
 WORKDIR /app
@@ -10,4 +10,14 @@ RUN yarn --frozen-lockfile && \
 RUN yarn install --production --ignore-scripts --prefer-offline
 RUN yarn cache clean
 
+FROM node:12.18-alpine
+
+COPY . /app
+WORKDIR /app
+
+# copy from build image
+COPY --from=BUILD_IMAGE /app/dist ./dist
+COPY --from=BUILD_IMAGE /app/.next ./.next
+COPY --from=BUILD_IMAGE /app/node_modules ./node_modules
+
 CMD yarn start