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/start.sh

@@ -205,4 +205,4 @@ sed -i "s!__LND_REST_API_URL__!${__LND_REST_API_URL__}!g" mempool-config.json
 # CLN
 sed -i "s!__CLN_SOCKET__!${__CLN_SOCKET__}!g" mempool-config.json
 
-node /backend/dist/index.js
+node /backend/package/index.js