This post works with Raspbian 10 buster
on CPU ARMv7 rev 3 (v7l) @ 4x 1.5GHz
, model Raspberry Pi 4. It solves the problem of cannot adding apt repositories for Raspbian Buster
when following Docker Official Installation Docs, issues may happen at here.
$ sudo add-apt-repository \
"deb [arch=amd64] https://download.docker.com/linux/debian \
$(lsb_release -cs) \
stable"
Solving
Successfully install Docker in Raspberry Pi 4

As for solving the problem, we need to install it from .deb
files which are located in these URLs from Official
https://download.docker.com/linux/debian/dists/buster/pool/stable/armhf/containerd.io_1.2.6-3_armhf.deb
https://download.docker.com/linux/debian/dists/buster/pool/stable/armhf/docker-ce-cli_18.09.7~3-0~debian-buster_armhf.deb
https://download.docker.com/linux/debian/dists/buster/pool/stable/armhf/docker-ce_18.09.7~3-0~debian-buster_armhf.deb
So you need to install the stretch packages on Buster. It turns out that the Docker packages for Raspbian Stretch actually work without modifications on Raspbian Buster, so we can just use those and install them using dpkg
.
wget https://download.docker.com/linux/debian/dists/buster/pool/stable/armhf/containerd.io_1.2.6-3_armhf.deb
wget https://download.docker.com/linux/debian/dists/buster/pool/stable/armhf/docker-ce-cli_18.09.7~3-0~debian-buster_armhf.deb
wget https://download.docker.com/linux/debian/dists/buster/pool/stable/armhf/docker-ce_18.09.7~3-0~debian-buster_armhf.deb
dpkg -i containerd.io_1.2.6-3_armhf.deb
dpkg -i docker-ce-cli_18.09.7~3-0~debian-buster_armhf.deb
dpkg -i docker-ce_18.09.7~3-0~debian-buster_armhf.deb
sudo usermod pi -aG docker
That’s it, now log out and log in again.
Test It!

It works!
Installing CUPS
I use docker to run CUPS
in Raspberry Pi 4 as a Printer Server. Reference to olbat/cupsd. Use this script. Mapping container’s 631 port to Pi’s 631 port
docker run -d -p 631:631 -v /var/run/dbus:/var/run/dbus --name cupsd olbat/cupsd
Then the image is downloading, and running successfully.