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.)

backend/package.json

@@ -24,6 +24,8 @@
     "tsc": "./node_modules/typescript/bin/tsc -p tsconfig.build.json",
     "build": "npm run tsc && npm run create-resources",
     "create-resources": "cp ./src/tasks/price-feeds/mtgox-weekly.json ./dist/tasks && node dist/api/fetch-version.js",
+    "package": "npm run build && rm -rf package && mv dist package && mv node_modules package && npm run package-rm-build-deps",
+    "package-rm-build-deps": "(cd package/node_modules; rm -r typescript @typescript-eslint)",
     "start": "node --max-old-space-size=2048 dist/index.js",
     "start-production": "node --max-old-space-size=4096 dist/index.js",
     "test": "./node_modules/.bin/jest --coverage",