backend: Fetch package version at build time

Extract `fetch-version.ts` which is called at build time to create
file `dist/api/version.json`.
This file is read by `backend-info.ts` at runtime.

This also fixes handing over the Git commit hash to the backend app
in the Docker backend image, which was broken as of 2022-07-12.
(Reason: The commit hash was previously required at runtime, but was
only provided at build time.)

backend/package.json

@@ -22,7 +22,8 @@
   "main": "index.ts",
   "scripts": {
     "tsc": "./node_modules/typescript/bin/tsc -p tsconfig.build.json",
-    "build": "npm run tsc && cp ./src/tasks/price-feeds/mtgox-weekly.json ./dist/tasks/",
+    "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",
     "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",