Unable to build multi architecture docker image

I want to built a docker image on my Intel laptop. The image will be deployed on both x86/x64 and arm64 hosts. The docker file uses ubuntu:20.04 base image. I tried building the image with following command:

docker buildx build . --no-cache --progress plain --platform linux/arm64/v8,linux/amd64 -t mytag

But it fails with error:

#10 [linux/arm64  3/17] RUN apt update &&     apt install -y --no-install-recommends software-properties-common &&     add-apt-repository -y ppa:ubuntu-toolchain-r/test &&     add-apt-repository -y "deb http://security.ubuntu.com/ubuntu xenial-security main" &&     add-apt-repository -y ppa:deadsnakes/ppa  &&     apt-get update &&     apt-get install -y --no-install-recommends     sudo     usbutils     libcanberra-gtk3-module ...*#10 191.3 Reading package lists...#10 195.6 E: Failed to fetch http://security.ubuntu.com/ubuntu/dists/xenial-security/main/binary-arm64/Packages  404  Not Found [IP: 185.125.190.39 80]#10 195.6 E: Some index files failed to download. They have been ignored, or old ones used instead.#10 ERROR: process "/bin/sh -c apt update &&     apt install -y --no-install-recommends software-properties-common &&     add-apt-repository -y ppa:ubuntu-toolchain-r/test &&     add-apt-repository -y \"deb http://security.ubuntu.com/ubuntu xenial-security main\" &&     add-apt-repository -y ppa:deadsnakes/ppa  &&     apt-get update &&     apt-get install -y --no-install-recommends     sudo     usbutils     libcanberra-gtk3-module ..." did not complete successfully: exit code: 100

Referring to this answer, I added following lines to my Dockerfile:

## add arm64 architectureRUN apt-get updateRUN dpkg --add-architecture arm64## arch-qualify the current repositoriesRUN sed -i "s/deb h/deb [arch=amd64] h/g" /etc/apt/sources.list## add arm64's reposRUN echo "# arm64 repositories" >> /etc/apt/sources.listRUN echo "deb [arch=arm64] http://ports.ubuntu.com/ubuntu-ports focal main restricted" >> /etc/apt/sources.listRUN echo "deb [arch=arm64] http://ports.ubuntu.com/ubuntu-ports focal-updates main restricted" >> /etc/apt/sources.listRUN echo "deb [arch=arm64] http://ports.ubuntu.com/ubuntu-ports focal universe" >> /etc/apt/sources.listRUN echo "deb [arch=arm64] http://ports.ubuntu.com/ubuntu-ports focal-updates universe" >> /etc/apt/sources.listRUN echo "deb [arch=arm64] http://ports.ubuntu.com/ubuntu-ports focal multiverse" >> /etc/apt/sources.listRUN echo "deb [arch=arm64] http://ports.ubuntu.com/ubuntu-ports focal-updates multiverse" >> /etc/apt/sources.listRUN echo "deb [arch=arm64] http://ports.ubuntu.com/ubuntu-ports focal-backports main restricted universe multiverse" >> /etc/apt/sources.listRUN echo "deb [arch=arm64] http://ports.ubuntu.com/ubuntu-ports focal-security main restricted" >> /etc/apt/sources.listRUN echo "deb [arch=arm64] http://ports.ubuntu.com/ubuntu-ports focal-security universe" >> /etc/apt/sources.listRUN echo "deb [arch=arm64] http://ports.ubuntu.com/ubuntu-ports focal-security multiverse" >> /etc/apt/sources.list

But now it gives me following error:

#23 19.23 E: Failed to fetch http://ports.ubuntu.com/ubuntu-ports/dists/focal/main/binary-amd64/Packages  404  Not Found [IP: 185.125.190.39 80]#23 19.23 E: Failed to fetch http://ports.ubuntu.com/ubuntu-ports/dists/focal-updates/multiverse/binary-amd64/Packages  404  Not Found [IP: 185.125.190.39 80]#23 19.23 E: Failed to fetch http://ports.ubuntu.com/ubuntu-ports/dists/focal-backports/main/binary-amd64/Packages  404  Not Found [IP: 185.125.190.39 80]#23 19.23 E: Failed to fetch http://ports.ubuntu.com/ubuntu-ports/dists/focal-security/multiverse/binary-amd64/Packages  404  Not Found [IP: 185.125.190.39 80]#23 19.23 E: Some index files failed to download. They have been ignored, or old ones used instead.#23 19.23 W: Target Packages (main/binary-all/Packages) is configured multiple times in /etc/apt/sources.list:3 and /etc/apt/sources.list:51#23 19.23 W: Target Packages (restricted/binary-all/Packages) is configured multiple times in /etc/apt/sources.list:3 and /etc/apt/sources.list:51#23 19.24 W: Target Packages (universe/binary-all/Packages) is configured multiple times in /etc/apt/sources.list:14 and /etc/apt/sources.list:53#23 19.24 W: Target Packages (multiverse/binary-all/Packages) is configured multiple times in /etc/apt/sources.list:24 and /etc/apt/sources.list:55#23 19.24 W: Target Packages (main/binary-all/Packages) is configured multiple times in /etc/apt/sources.list:8 and /etc/apt/sources.list:52#23 19.24 W: Target Packages (restricted/binary-all/Packages) is configured multiple times in /etc/apt/sources.list:8 and /etc/apt/sources.list:52#23 19.24 W: Target Packages (universe/binary-all/Packages) is configured multiple times in /etc/apt/sources.list:16 and /etc/apt/sources.list:54#23 19.24 W: Target Packages (multiverse/binary-all/Packages) is configured multiple times in /etc/apt/sources.list:26 and /etc/apt/sources.list:56#23 19.24 W: Target Packages (main/binary-all/Packages) is configured multiple times in /etc/apt/sources.list:34 and /etc/apt/sources.list:57#23 19.24 W: Target Packages (restricted/binary-all/Packages) is configured multiple times in /etc/apt/sources.list:34 and /etc/apt/sources.list:57#23 19.24 W: Target Packages (universe/binary-all/Packages) is configured multiple times in /etc/apt/sources.list:34 and /etc/apt/sources.list:57#23 19.24 W: Target Packages (multiverse/binary-all/Packages) is configured multiple times in /etc/apt/sources.list:34 and /etc/apt/sources.list:57#23 19.24 W: Target Packages (main/binary-all/Packages) is configured multiple times in /etc/apt/sources.list:44 and /etc/apt/sources.list:58#23 19.24 W: Target Packages (restricted/binary-all/Packages) is configured multiple times in /etc/apt/sources.list:44 and /etc/apt/sources.list:58#23 19.24 W: Target Packages (universe/binary-all/Packages) is configured multiple times in /etc/apt/sources.list:46 and /etc/apt/sources.list:59#23 19.24 W: Target Packages (multiverse/binary-all/Packages) is configured multiple times in /etc/apt/sources.list:48 and /etc/apt/sources.list:60#23 ERROR: process "/bin/sh -c apt-get update" did not complete successfully: exit code: 100#9 [linux/amd64  2/31] RUN apt-get update#9 39.27 Get:16 http://archive.ubuntu.com/ubuntu focal-updates/main amd64 Packages [3553 kB]#9 CANCELED------> [linux/arm64 16/31] RUN apt-get update:19.24 W: Target Packages (universe/binary-all/Packages) is configured multiple times in /etc/apt/sources.list:16 and /etc/apt/sources.list:5419.24 W: Target Packages (multiverse/binary-all/Packages) is configured multiple times in /etc/apt/sources.list:26 and /etc/apt/sources.list:5619.24 W: Target Packages (main/binary-all/Packages) is configured multiple times in /etc/apt/sources.list:34 and /etc/apt/sources.list:5719.24 W: Target Packages (restricted/binary-all/Packages) is configured multiple times in /etc/apt/sources.list:34 and /etc/apt/sources.list:5719.24 W: Target Packages (universe/binary-all/Packages) is configured multiple times in /etc/apt/sources.list:34 and /etc/apt/sources.list:5719.24 W: Target Packages (multiverse/binary-all/Packages) is configured multiple times in /etc/apt/sources.list:34 and /etc/apt/sources.list:5719.24 W: Target Packages (main/binary-all/Packages) is configured multiple times in /etc/apt/sources.list:44 and /etc/apt/sources.list:5819.24 W: Target Packages (restricted/binary-all/Packages) is configured multiple times in /etc/apt/sources.list:44 and /etc/apt/sources.list:5819.24 W: Target Packages (universe/binary-all/Packages) is configured multiple times in /etc/apt/sources.list:46 and /etc/apt/sources.list:5919.24 W: Target Packages (multiverse/binary-all/Packages) is configured multiple times in /etc/apt/sources.list:48 and /etc/apt/sources.list:60------WARNING: No output specified with docker-container driver. Build result will only remain in the build cache. To push result image into registry use --push or to load image into docker use --loadDockerfile:31--------------------  29 |     RUN echo "deb [arch=arm64] http://ports.ubuntu.com/ubuntu-ports focal-security multiverse" >> /etc/apt/sources.list  30 |       31 | >>> RUN apt-get update  32 |     # Set environment variables to avoid interactive prompts during installation  33 |     ENV DEBIAN_FRONTEND=noninteractive--------------------ERROR: failed to solve: process "/bin/sh -c apt-get update" did not complete successfully: exit code: 100

What I am missing here?

Update 1

I forgot to tell that I was able to successfully build the image for the same architecture as my laptop (x86/x64 or amd64) with command:

docker build . --no-cache --progress plain -t mytag

The issue started when I tried to build for arm64 / multi-architecture.

Update 2

I tried by commenting lines corresponding to 404 error:

# RUN echo "deb [arch=arm64] http://ports.ubuntu.com/ubuntu-ports focal main restricted" >> /etc/apt/sources.list   # RUN echo "deb [arch=arm64] http://ports.ubuntu.com/ubuntu-ports focal-updates multiverse" >> /etc/apt/sources.list# RUN echo "deb [arch=arm64] http://ports.ubuntu.com/ubuntu-ports focal-backports main restricted universe multiverse" >> /etc/apt/sources.list# RUN echo "deb [arch=arm64] http://ports.ubuntu.com/ubuntu-ports focal-security multiverse" >> /etc/apt/sources.list

But still same error.

This was expected, as the 404 is occuring for amd64 packages and not arm64 packages:

#23 19.23 E: Failed to fetch http://ports.ubuntu.com/ubuntu-ports/dists/focal/main/binary-amd64/Packages  404  Not Found [IP: 185.125.190.39 80]#23 19.23 E: Failed to fetch http://ports.ubuntu.com/ubuntu-ports/dists/focal-updates/multiverse/binary-amd64/Packages  404  Not Found [IP: 185.125.190.39 80]#23 19.23 E: Failed to fetch http://ports.ubuntu.com/ubuntu-ports/dists/focal-backports/main/binary-amd64/Packages  404  Not Found [IP: 185.125.190.39 80]#23 19.23 E: Failed to fetch http://ports.ubuntu.com/ubuntu-ports/dists/focal-security/multiverse/binary-amd64/Packages  404  Not Found [IP: 185.125.190.39 80]

But I am guessing why it is failing for amd64 packages as it used to successfully build the image before I added changes for arm64.