With Ubuntu on arm64 apt/sources.list.d/docker.list specifies wrong architecture by default

What you expected to happen?

On a Raspberry Pi 3 with Ubuntu 18.04 I expected this to be able to install Docker.

What happened?

Docker fails to install. The base error is that /etc/apt/sources.list.d/docker.list specifies the wrong architecture:

# This file is managed by Puppet. DO NOT EDIT.
# docker
deb [arch=aarch64] https://download.docker.com/linux/ubuntu bionic stable

For apt the correct architecture for 64-bit ARM is arm64.

How to reproduce it?

class { 'docker':
  version => '5:18.09.6~3-0~ubuntu-bionic',
}

Anything else we need to know?

To workaround this I have to add:

class { 'docker::repos':
  architecture => 'arm64',
}

docker::repos gets the default value of architecture from $facts['architecture']. Looking through all the facts returned on a Raspberry Pi 3 I cannot see any fact returning arm64. So I'm not sure what the correct fact would be or how other modules handle this.

Versions:

$ puppet --version
5.4.0

$ docker version
Client:
 Version:           18.09.6
 API version:       1.39
 Go version:        go1.10.8
 Git commit:        481bc77
 Built:             Sat May  4 02:40:48 2019
 OS/Arch:           linux/arm64
 Experimental:      false

Server: Docker Engine - Community
 Engine:
  Version:          18.09.6
  API version:      1.39 (minimum version 1.12)
  Go version:       go1.10.8
  Git commit:       481bc77
  Built:            Sat May  4 02:00:10 2019
  OS/Arch:          linux/arm64
  Experimental:     false

$ facter os
{
  architecture => "aarch64",
  distro => {
    codename => "bionic",
    description => "Ubuntu 18.04.2 LTS",
    id => "Ubuntu",
    release => {
      full => "18.04",
      major => "18.04"
    }
  },
  family => "Debian",
  hardware => "aarch64",
  name => "Ubuntu",
  release => {
    full => "18.04",
    major => "18.04"
  },
  selinux => {
    enabled => false
  }
}

$ puppet module list
/usr/share/puppet/modules (no modules installed)

Logs:

Run with --debug here

Start at the docker::repos architecture default and the generated /etc/apt/sources.list.d/docker.list, then compare Facter's aarch64 value with apt's required arm64 value. Done means a Raspberry Pi 3 on Ubuntu 18.04 can install Docker without setting the architecture override; an open linked pull request already tracks this work.