Add installation of libavif15 from bookworm-backports for Prince 16.x compatibility

Author: max-ostapenko

cloudbuild.yaml

@@ -80,6 +80,11 @@ steps:
         # Install pdftk (ebook post-processing)
         apt-get install -y pdftk
 
+        # Prince 16.x requires libavif15, which is only in bookworm-backports
+        echo "deb http://deb.debian.org/debian bookworm-backports main" > /etc/apt/sources.list.d/backports.list
+        apt-get update -qq
+        apt-get install -y -t bookworm-backports libavif15
+
         # Download and install Prince PDF generator
         wget -q "https://www.princexml.com/download/${_PRINCE_PACKAGE}" -P /tmp
         apt-get install -y "/tmp/${_PRINCE_PACKAGE}"