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>

app.js

@@ -27,8 +27,9 @@ app.set('view engine', 'pug');
 app.use(express.urlencoded({ extended: true }));
 app.use(express.json());
 
-// Static reports
-app.use('/reports', express.static(join(__dirname, 'reports')));
+// Static reports — use REPORTS_DIR env var so Docker and local both work
+const REPORTS_DIR = process.env.REPORTS_DIR || join(__dirname, 'reports');
+app.use('/reports', express.static(REPORTS_DIR));
 
 // Routes
 app.use('/', indexRouter);