docker: use arm64 instead of aarch64

If architecture is "aarch64" we should use the string "arm64" with the
Docker repo.

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={{ ansible_architecture }}] https://download.docker.com/linux/{{ ansible_distribution | lower }} {{ ansible_distribution_release }} stable"
+    repo: "deb [arch={% if {{ ansible_architecture }} == aarch64 %}arm64 {% endif %}] https://download.docker.com/linux/{{ ansible_distribution | lower }} {{ ansible_distribution_release }} stable"
     update_cache: yes
 
 - name: Install Docker CE