fix(docker): add libavif-dev for AVIF image upload support

Pillow 11 supports AVIF natively but requires libavif to be installed
at the system level. Without it, uploading AVIF images via Wagtail's
image chooser causes an unhandled 500 error.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Dockerfile

@@ -18,6 +18,7 @@ RUN set -eux; \
         apt-get install -y --no-install-recommends \
             build-essential \
             libpq-dev \
+            libavif-dev \
             curl \
             nodejs \
             npm \