These won't be exact instructions, since I decided to switch to focal (mostly for other reasons).
mkdir zfs-scratch
cd zfs-scratch
apt-get download linux-headers-current-rockchip64
git clone -b zfs-0.8.5 https://github.com/openzfs/zfs.git
docker run --rm -it -v $(pwd):/scratch ubuntu:focal
# inside the container
cd /scratch
apt update
apt install build-essential autoconf automake bison flex libtool gawk alien fakeroot dkms libblkid-dev uuid-dev libudev-dev libssl-dev zlib1g-dev libaio-dev libattr1-dev libelf-dev python3 python3-dev python3-setuptools python3-cffi libffi-dev
dpkg -i linux-headers-current-*.deb
cd zfs
sh autogen.sh
./configure
make -s -j$(nproc) deb
At that point you can exit the container (it'l vanish because of the --rm) and inside zfs-scratch/zfs you should have a bunch of Debian packages you can install
