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/src/api/fetch-version.ts

@@ -0,0 +1,37 @@
+import fs from 'fs';
+import path from "path";
+const { spawnSync } = require('child_process');
+
+function getVersion(): string {
+  const packageJson = fs.readFileSync('package.json').toString();
+  return JSON.parse(packageJson).version;
+}
+
+function getGitCommit(): string {
+  if (process.env.DOCKER_COMMIT_HASH) {
+    return process.env.DOCKER_COMMIT_HASH;
+  } else {
+    const gitRevParse = spawnSync('git', ['rev-parse', '--short', 'HEAD']);
+    if (!gitRevParse.error) {
+      const output = gitRevParse.stdout.toString('utf-8').replace(/[\n\r\s]+$/, '');
+      if (output) {
+        return output;
+      } else {
+        console.log('Could not fetch git commit: No repo available');
+      }
+    } else if (gitRevParse.error.code === 'ENOENT') {
+      console.log('Could not fetch git commit: Command `git` is unavailable');
+    }
+  }
+  return '?';
+}
+
+const versionInfo = {
+  version: getVersion(),
+  gitCommit: getGitCommit()
+}
+
+fs.writeFileSync(
+  path.join(__dirname, 'version.json'),
+  JSON.stringify(versionInfo, null, 2) + "\n"
+);