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>

start.sh

@@ -0,0 +1,12 @@
+#!/bin/bash
+set -e
+
+# Start virtual framebuffer so Chrome/Firefox have a display to render into.
+# This is what the base sitespeedio/sitespeed.io image's entrypoint does.
+Xvfb :99 -ac -screen 0 1920x1080x24 +extension RANDR &
+export DISPLAY=:99
+
+# Give Xvfb a moment to initialise
+sleep 1
+
+exec node /app/app.js