Do not hardcode amd64 in setup_debian.yml

Until now, we've only supported non-amd64 on Raspbian.

Seems like there are now people running Debian/Ubuntu on ARM,
so we were forcing them into amd64 Docker packages.

I've gotten a report that this change fixes support
for Ubuntu Server 20.04 on RPi 4B.

roles/matrix-base/defaults/main.yml

@@ -29,6 +29,11 @@ matrix_federation_public_port: 8448
 # See docs/alternative-architectures.md
 matrix_architecture: amd64
 
+# The architecture for Debian packages.
+# See: https://wiki.debian.org/SupportedArchitectures
+# We just remap from our `matrix_architecture` values to what Debian and possibly other distros call things.
+matrix_debian_arch: "{{ 'armhf' if matrix_architecture == 'arm32' else matrix_architecture }}"
+
 matrix_user_username: "matrix"
 matrix_user_groupname: "matrix"
 

roles/matrix-base/tasks/server_base/setup_debian.yml

@@ -20,7 +20,7 @@
 
 - name: Ensure Docker repository is enabled
   apt_repository:
-    repo: "deb [arch=amd64] https://download.docker.com/linux/{{ ansible_distribution|lower }} {{ ansible_distribution_release }} stable"
+    repo: "deb [arch={{ matrix_debian_arch }}] https://download.docker.com/linux/{{ ansible_distribution|lower }} {{ ansible_distribution_release }} stable"
     state: present
     update_cache: yes
   when: matrix_docker_installation_enabled|bool and matrix_docker_package_name == 'docker-ce'