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>

Dockerfile

@@ -17,6 +17,8 @@ RUN npm install --omit=dev
 
 COPY . .
 
+RUN chmod +x /app/start.sh
+
 # Create persistent directories
 RUN mkdir -p /data/reports
 
@@ -33,8 +35,6 @@ ENV PORT=3132 \
 
 EXPOSE 3132
 
-# Override the base image's ENTRYPOINT (which is "sitespeed.io") so that
-# Docker doesn't prepend it to our CMD, causing sitespeed.io to treat
-# "node" as a URL to test.
-ENTRYPOINT ["node"]
-CMD ["app.js"]
+# start.sh boots Xvfb (virtual display for Chrome/Firefox) then runs the app.
+# This mirrors what the base image's own entrypoint does.
+ENTRYPOINT ["/app/start.sh"]