image-builder: add python3 (needed by batocera build-rootfs.sh)

distros/batocera/build-rootfs.sh:211 calls python3 inline to patch
libretroControllers.py with the PS5 HOTKEY-on-gamepad fix. Without it
the batocera build exits 127 at the very end of an otherwise-clean
run.

docker/image-builder/Dockerfile

@@ -9,6 +9,7 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
     parted dosfstools e2fsprogs kmod \
     initramfs-tools fdisk gdisk udev kpartx \
     xz-utils bzip2 zstd \
+    python3 \
     && rm -rf /var/lib/apt/lists/*
 
 # Install Go (Ubuntu 24.04 only has 1.22, distrobuilder needs newer)