update Dockerfiles based on official example

Dockerfile

@@ -1,26 +1,41 @@
-FROM node:14-alpine as builder
-
+# Install dependencies only when needed
+FROM node:14-alpine AS deps
+# Check https://github.com/nodejs/docker-node/tree/b4117f9333da4138b03a546ec926ef50a31506c3#nodealpine to understand why libc6-compat might be needed.
+RUN apk add --no-cache libc6-compat
+WORKDIR /app
 COPY package.json package-lock.json ./
-RUN npm install
+RUN npm ci
 
-COPY tsconfig.json next-env.d.ts .babelrc next.config.js next-sitemap.js ./
-COPY src src/
-COPY public public/
-COPY types types/
+# Rebuild the source code only when needed
+FROM node:14-alpine AS builder
+WORKDIR /app
+COPY . .
+COPY --from=deps /app/node_modules ./node_modules
 
 ENV NEXT_TELEMETRY_DISABLED=1
 ENV NEXT_PUBLIC_API_URL=https://api.dev.sahkoinsinoorikilta.fi/api
 ENV NEXT_PUBLIC_SITE_URL=https://dev.sahkoinsinoorikilta.fi
 RUN npm run build
 
-FROM node:14-alpine as server
-WORKDIR /www
-COPY package.json package-lock.json next.config.js next-sitemap.js ./
-COPY --from=builder .next .next
-COPY --from=builder node_modules node_modules
-COPY --from=builder public public
+# Production image, copy all the files and run next
+FROM node:14-alpine AS runner
+WORKDIR /app
+
+ENV NODE_ENV production
+
+COPY --from=builder /app/next.config.js ./
+COPY --from=builder /app/public ./public
+COPY --from=builder /app/.next ./.next
+COPY --from=builder /app/node_modules ./node_modules
+COPY --from=builder /app/package.json ./package.json
 
 RUN npm prune --production
 
+RUN addgroup -g 1001 -S nodejs
+RUN adduser -S nextjs -u 1001
+RUN chown -R nextjs:nodejs /app/.next
+USER nextjs
+
 EXPOSE 3000
-ENTRYPOINT ["npm", "run", "serve"]
+
+CMD ["npm", "run", "serve"]