Install rpi-update on docker image before trying to run it

.gitlab-ci.yml

@@ -39,6 +39,7 @@ build-zynthbox-image-armhf:
     - sudo mount --bind /proc $mountdir/proc
     - sudo mount --bind /sys $mountdir/sys
     - echo "# Updating firmware and bootloader"
+    - sudo curl -L --output /usr/bin/rpi-update https://raw.githubusercontent.com/raspberrypi/rpi-update/master/rpi-update && sudo chmod +x /usr/bin/rpi-update
     - sudo SKIP_WARNING=1 ROOT_PATH=$mountdir BOOT_PATH=$mountdir/boot rpi-update
     # - echo "# Cleaning up old and unused kernels"
     # - sudo chroot $mountdir apt -yy remove "linux-headers-6.6.2*" "linux-image-6.6.2*" "linux-headers-rpi-v*" "linux-image-rpi-v*" "linux-kbuild-6.6.2*"