fix: start Xvfb before Node so Chrome has a display

Without Xvfb running, Chrome exits immediately with code 1.
The base sitespeedio image normally handles this via its own
entrypoint script; we now replicate that with start.sh.
Also drop --headless (breaks visual metrics) and add --disable-gpu
for stable Chrome rendering inside Docker.

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

runner.js

@@ -22,17 +22,18 @@ export function runTest(job, onLine) {
       '--sustainable.enable',
       '--axe.enable',
       '--coach',
-      '--headless',
     ];
 
     if (job.mobile) {
       args.push('--mobile');
     }
 
-    // In Docker we need --xvfb false and --browsertime.chrome.args=no-sandbox
+    // Chrome needs these flags when running inside Docker
     if (process.env.IN_DOCKER) {
       args.push('--browsertime.chrome.args', 'no-sandbox');
       args.push('--browsertime.chrome.args', 'disable-dev-shm-usage');
+      // Disable GPU compositing — not needed in Xvfb and can cause crashes
+      args.push('--browsertime.chrome.args', 'disable-gpu');
     }
 
     onLine(`[runner] Starting: node ${args.slice(0, 3).join(' ')} ...`);