Fix Quarto cache permission: create /var/www/.cache/quarto for www-data

Quarto resolves its cache dir as $HOME/.cache/quarto. The process runs
as www-data (home=/var/www) but that directory is root-owned, so Quarto
crashed immediately with PermissionDenied on mkdir. Pre-create the cache
dir and chown it to www-data at image build time.

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

server-ce/Dockerfile-base

@@ -40,7 +40,9 @@ RUN --mount=type=cache,target=/var/cache/apt,sharing=locked \
 ARG QUARTO_VERSION=1.6.39
 RUN curl -fsSL "https://github.com/quarto-dev/quarto-cli/releases/download/v${QUARTO_VERSION}/quarto-${QUARTO_VERSION}-linux-amd64.deb" -o /tmp/quarto.deb \
 &&  dpkg -i /tmp/quarto.deb \
-&&  rm /tmp/quarto.deb
+&&  rm /tmp/quarto.deb \
+&&  mkdir -p /var/www/.cache/quarto \
+&&  chown -R www-data:www-data /var/www/.cache
 
 
 # Set up overleaf user and home directory