backend: Create npm script package

This script creates a directory `backend/package` which only contains the
files required by the backend at runtime:
- The contents of `dist`
- `node_modules` minus `typescript` and `@typescript-eslint`.
  These packages are build-only and are larger than the remaining whole package.

By using only `backend/package` in the Docker image,
the backend content size in the image is decreased by 70% to 31M.
This, along with the improved copying in the Dockerfile, reduces the
backend image size by 44% to 200M.
(Step `RUN chown -R 1000:1000 /backend ...` created a layer that effectively
duplicated the backend.)

docker/backend/Dockerfile

@@ -9,18 +9,15 @@ COPY . .
 RUN apt-get update
 RUN apt-get install -y build-essential python3 pkg-config
 RUN npm install --omit=dev --omit=optional
-RUN npm run build
+RUN npm run package
 
 FROM node:16.16.0-buster-slim
 
 WORKDIR /backend
 
-COPY --from=builder /build/ .
-
-RUN chmod +x /backend/start.sh
-RUN chmod +x /backend/wait-for-it.sh
-
-RUN chown -R 1000:1000 /backend && chmod -R 755 /backend
+RUN chown 1000:1000 ./
+COPY --from=builder --chown=1000:1000 /build/package ./package/
+COPY --from=builder --chown=1000:1000 /build/mempool-config.json /build/start.sh /build/wait-for-it.sh ./
 
 USER 1000