Ubuntu helper images generated all platforms are of x86 arch

Summary

The commit has added Ubuntu helper images for all the platforms. However the images generated for all platforms seems to be x86_64 images. All the ubuntu images are using amd64 dependencies instead using the arch-specific dependencies.

Steps to reproduce

Helper image job is responsible for creating all the alpine and ubuntu helper images. Reference job: https://gitlab.com/gitlab-org/gitlab-runner/-/jobs/1427096009

All the helper images are present in this artefacts directory. You can test ubuntu images for any arch other than x86_64. Download it on your machine and use below commands to test it:

$ docker import prebuilt-ubuntu-s390x.tar.xz ubuntu-s390x-test:latest
sha256:8dceacb2306054b033aecee0ec60253c02fa2c6f96b29153c09ea3972eee5d81

$ docker run --rm ubuntu-s390x-test:latest arch
x86_64

Actual behavior

The Ubuntu based helper images created for platforms other than linux/amd64 are not appropriate as all the images seem to be of x86 architecture.

Expected behavior

The Ubuntu based helper images created for each platform should work on respective platform.

Eg: Ubuntu image for s390x arch should give below output:

$ docker run --rm ubuntu-s390x-test:latest arch
s390x

Relevant logs and/or screenshots

Adding a snippet of log from helper-image job for reference. As you can see that the image dowloads all the amd64 dependencies instead of s390x.

job log

Building helper image for: linux/s390x based on ubuntu:20.04
docker-buildx
Successfully created context "docker-buildx"
buildx-builder
#1 [internal] booting buildkit
#1 pulling image moby/buildkit:buildx-stable-1
#1 pulling image moby/buildkit:buildx-stable-1 0.3s done
#1 creating container buildx_buildkit_buildx-builder0
#1 creating container buildx_buildkit_buildx-builder0 0.5s done
#1 DONE 0.8s
#2 [internal] load build definition from Dockerfile.ubuntu
#2 transferring dockerfile: 954B done
#2 DONE 0.1s
#3 [internal] load .dockerignore
#3 transferring context: 2B done
#3 DONE 0.0s
#4 [internal] load metadata for docker.io/library/ubuntu:20.04
#4 DONE 0.6s
#8 [internal] load build context
#8 transferring context: 29.07MB 0.8s done
#8 DONE 0.8s
#5 [1/6] FROM docker.io/library/ubuntu:20.04@sha256:b3e2e47d016c08b3396b5eb...
#5 resolve docker.io/library/ubuntu:20.04@sha256:b3e2e47d016c08b3396b5ebe06ab0b711c34e7f37b98c9d37abe794b71cea0a2 0.0s done
#5 sha256:a31c7b29f4ad2bd946738970f040825704a523ded1b9d6f9c7c7cafc6ab731df 28.57MB / 28.57MB 0.5s done
#5 extracting sha256:a31c7b29f4ad2bd946738970f040825704a523ded1b9d6f9c7c7cafc6ab731df
#5 extracting sha256:a31c7b29f4ad2bd946738970f040825704a523ded1b9d6f9c7c7cafc6ab731df 1.0s done
#5 DONE 1.5s
#6 [2/6] RUN apt-get update && apt-get install -y bash ca-certificates dumb...
#6 0.152 Get:1 http://security.ubuntu.com/ubuntu focal-security InRelease [114 kB]
#6 0.321 Get:2 http://security.ubuntu.com/ubuntu focal-security/restricted amd64 Packages [368 kB]
#6 0.374 Get:3 http://archive.ubuntu.com/ubuntu focal InRelease [265 kB]
#6 0.400 Get:4 http://security.ubuntu.com/ubuntu focal-security/multiverse amd64 Packages [27.6 kB]
#6 0.402 Get:5 http://security.ubuntu.com/ubuntu focal-security/main amd64 Packages [930 kB]
#6 0.435 Get:6 http://security.ubuntu.com/ubuntu focal-security/universe amd64 Packages [779 kB]
#6 0.880 Get:7 http://archive.ubuntu.com/ubuntu focal-updates InRelease [114 kB]
#6 0.996 Get:8 http://archive.ubuntu.com/ubuntu focal-backports InRelease [101 kB]
#6 1.116 Get:9 http://archive.ubuntu.com/ubuntu focal/universe amd64 Packages [11.3 MB]
#6 1.717 Get:10 http://archive.ubuntu.com/ubuntu focal/main amd64 Packages [1275 kB]