How to build raspbian with custom kernel 5.10 (PREEMPT RT) - linux-kernel

Question

I am trying to enable PREEMPT RT (Fully preemptive model) in Linux kernel 5.10. However, I get a black screen when booting from the custom kernel image and I have no clue why that happens. Any help is greatly appreciated. Here are all my steps:
All below files are available at https://github.com/remusmp/rpi-rt-kernel. Just clone and run make.
I build with docker. Here is my Dockerfile:
FROM ubuntu:20.04
ENV TZ=Europe/Copenhagen
RUN ln -snf /usr/share/zoneinfo/$TZ /etc/localtime && echo $TZ > /etc/timezone
RUN apt-get update
RUN apt-get install -y git make gcc bison flex libssl-dev bc ncurses-dev kmod
RUN apt-get install -y crossbuild-essential-arm64
RUN apt-get install -y wget zip unzip fdisk nano
WORKDIR /rpi-kernel
RUN git clone https://github.com/raspberrypi/linux.git -b rpi-5.10.y --depth=1
RUN wget https://mirrors.edge.kernel.org/pub/linux/kernel/projects/rt/5.10/patch-5.10.59-rt52.patch.gz
WORKDIR /rpi-kernel/linux/
ENV KERNEL=kernel8
ENV ARCH=arm64
ENV CROSS_COMPILE=aarch64-linux-gnu-
RUN gzip -cd ../patch-5.10.59-rt52.patch.gz | patch -p1 --verbose
RUN make bcm2711_defconfig
ADD .config ./
RUN make Image modules dtbs
WORKDIR /raspios
RUN apt -y install
RUN wget https://downloads.raspberrypi.org/raspios_lite_armhf/images/raspios_lite_armhf-2021-05-28/2021-05-07-raspios-buster-armhf-lite.zip
RUN unzip 2021-05-07-raspios-buster-armhf-lite.zip && rm 2021-05-07-raspios-buster-armhf-lite.zip
RUN mkdir /raspios/mnt && mkdir /raspios/mnt/disk && mkdir /raspios/mnt/boot
ADD build.sh ./
ADD config.txt ./
Script build.sh:
#!/bin/sh
mount -t ext4 -o loop,offset=$((532480*512)) 2021-05-07-raspios-buster-armhf-lite.img /raspios/mnt/disk
mount -t vfat -o loop,offset=$((8192*512)),sizelimit=$((524288*512)) 2021-05-07-raspios-buster-armhf-lite.img /raspios/mnt/boot
cd /rpi-kernel/linux/
make INSTALL_MOD_PATH=/raspios/mnt/disk modules_install
make INSTALL_DTBS_PATH=/raspios/mnt/boot dtbs_install
cd -
cp /rpi-kernel/linux/arch/arm64/boot/Image /raspios/mnt/boot/$KERNEL\_rt.img
cp /rpi-kernel/linux/arch/arm64/boot/dts/broadcom/*.dtb /raspios/mnt/boot/
cp /rpi-kernel/linux/arch/arm64/boot/dts/overlays/*.dtb* /raspios/mnt/boot/overlays/
cp /rpi-kernel/linux/arch/arm64/boot/dts/overlays/README /raspios/mnt/boot/overlays/
cp /raspios/config.txt /raspios/mnt/boot/
touch /raspios/mnt/boot/ssh
umount /raspios/mnt/disk
umount /raspios/mnt/boot
zip 2021-05-07-raspios-buster-armhf-lite.zip 2021-05-07-raspios-buster-armhf-lite.img
I got a custom .config file, which I copied from the container after running make menuconfig and enabling Fully PREEMPT RT. I had to disable KVM first in order for Full PREEMPT RT option to show up but that is ok for my use case. This is the only customization I do to the kernel, nothing else.
The outcome is a zipped sd card image that I then decompress and dd to an sdcard with command:
sudo dd if=2021-05-07-raspios-buster-armhf-lite.img of=/dev/mmcblk0 bs=1M status=progress
Am I missing something in the above process or where should I look to get a working custom Linux kernel image? I followed the guide on https://www.raspberrypi.org/documentation/computers/linux_kernel.html and it works fine if I set the vanilla kernel8.img in config.txt but my custom kernel with preempt rt enabled doesn't work.
Many thanks!

Answer 1

It almost worked. I was missing:
arm_64bit=1
in config.txt.