Raspbian with fully preemptive real-time kernel 5.10.
Clone this repository and run make. It will create a folder build with a zipped sdcard image.
Run make custom. You will get a shell to a container. Then run:
cd /rpi-rt-kernel/linux
make menuconfig
After exiting a .config file is created, which needs to be copied out of the container:
docker cp your-container:/rpi-rt-kernel/linux/.config .config
Exit the container and run make again to get a new sdcard image.