docker: add support for multiple architectures

amd64 was hardcoded into the docker repo string. now it will try to use
the host's reported architecture.

roles/gitlab-runner/tasks/docker.yml

@@ -11,7 +11,7 @@
 - name: Add Docker apt repo to souces.list
   apt_repository:
     codename: "{{ ansible_distribution_release }}"
-    repo: "deb [arch=amd64] https://download.docker.com/linux/debian {{ ansible_distribution_release }} stable"
+    repo: "deb [arch={{ ansible_architecture }}] https://download.docker.com/linux/{{ ansible_distribution | lower }} {{ ansible_distribution_release }} stable"
     update_cache: yes
 
 - name: Install Docker CE