fix: replace broken symlink with REPORTS_DIR env var

The ln -sf /data/reports /app/reports was creating the symlink INSIDE
/app/reports/ (since that dir already existed from COPY) instead of
replacing it. Result: sitespeed.io wrote to /app/reports/<id> and the
parser looked there too, but the volume was at /data/reports.

Fix: set REPORTS_DIR=/data/reports in Docker ENV and use it in both
runner.js (outputFolder) and app.js (static serving). No symlink needed.
Also add .dockerignore to exclude reports/, node_modules/, .git/.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

runner.js

@@ -5,11 +5,11 @@ import { existsSync } from 'fs';
 
 const __dirname = dirname(fileURLToPath(import.meta.url));
 const LOCAL_BIN = join(__dirname, '..', 'sitespeed.io', 'bin', 'sitespeed.js');
-
+const REPORTS_DIR = process.env.REPORTS_DIR || join(__dirname, 'reports');
 
 export function runTest(job, onLine) {
   return new Promise((resolve, reject) => {
-    const outputFolder = join(__dirname, 'reports', job.id);
+    const outputFolder = join(REPORTS_DIR, job.id);
     const isDocker = !!process.env.IN_DOCKER;
 
     const sitespeedArgs = [