Install x11-utils and wait reliably for Xvfb in entrypoint

The Xvfb readiness loop in entrypoint.sh used xdpyinfo, which isn't
in the Playwright base image. On busy hosts the loop fell through
before Xvfb was ready, so Chromium launched with no X server and
crashed with "Missing X server or \$DISPLAY".

Add x11-utils (provides xdpyinfo) and extend the readiness loop to
10 s with explicit logging.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Dockerfile

@@ -9,6 +9,7 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
     tzdata \
     xvfb \
     x11vnc \
+    x11-utils \
     novnc \
     websockify \
     fluxbox \