build(docker): declare fonts-urw-base35 explicitly

The image already ships these fonts, but only as a transitive dependency of
openbox (openbox -> libimlib2 -> libspectre -> libgs10 -> fonts-urw-base35).
They are what Verdana, Georgia, Trebuchet MS and Tahoma fall back to; without
them fontconfig resolves those families to the CJK font instead of a Latin one.

Listing the package directly means a future change to the window manager or
X stack cannot silently drop the Latin fallback. No runtime change.

Dockerfile

@@ -11,6 +11,7 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
     libgdk-pixbuf-2.0-0 libxss1 libxtst6 fonts-liberation \
     fonts-noto-color-emoji fonts-unifont fonts-freefont-ttf \
     fonts-ipafont-gothic fonts-wqy-zenhei fonts-tlwg-loma-otf \
+    fonts-urw-base35 \
     xvfb xdotool openbox \
     curl ca-certificates \
     && curl -fsSL https://deb.nodesource.com/setup_20.x | bash - \