Do not hardcode armhf for Raspbian

Raspbian doesn't seem to support arm64, so this is somewhat pointless
right now.

However, they might in the future. Doing this should also unify us
some more with `setup_debian.yml` with the ultimate goal of
eliminating `setup_raspbian.yml`.

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

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