Now everything works! :D

custom/main.sh

@@ -10,3 +10,5 @@ SCRIPTPATH="$( cd "$(dirname "$0")" >/dev/null 2>&1 ; pwd -P )"
 eval "${SCRIPTPATH}/install_packages.sh"
 eval "${SCRIPTPATH}/install_docker.sh"
 eval "${SCRIPTPATH}/install_docker-compose.sh"
+
+apt-get clean

preseed/preseed.cfg

@@ -464,4 +464,4 @@ d-i debian-installer/exit/poweroff boolean true
 # packages and run commands in the target system.
 #d-i preseed/late_command string apt-install zsh; in-target chsh -s /bin/zsh
 
-d-i preseed/late_command string cp -r /cdrom/custom /target/srv/custom || cp -r /hd-media/custom /target/srv/custom || echo "boop" > /target/srv/asd.txt
+d-i preseed/late_command string modprobe usb-storage isofs; mkdir -p /cdrom; mount /dev/cdrom /cdrom; cp -r /cdrom/custom /target/srv/custom

preseed/remote.cfg

@@ -17,4 +17,3 @@ d-i mirror/country string manual
 d-i mirror/http/hostname string debian.mirror.garr.it
 d-i mirror/http/directory string /debian
 d-i mirror/http/proxy string
-