docker: use ansible conditional for arch selection

in-line substitution is more tricky. this is a simple way to explicitly
declare the Docker apt repo string by architecture.

roles/gitlab-runner/tasks/docker.yml

@@ -8,11 +8,19 @@
     id: 9DC858229FC7DD38854AE2D88D81803C0EBFCD88
     url: https://download.docker.com/linux/debian/gpg
 
-- name: Add Docker apt repo to souces.list
+- name: Add arm64 Docker apt repo to souces.list
   apt_repository:
     codename: "{{ ansible_distribution_release }}"
-    repo: "deb [arch={% if {{ ansible_architecture }} == aarch64 %}arm64 {% endif %}] https://download.docker.com/linux/{{ ansible_distribution | lower }} {{ ansible_distribution_release }} stable"
+    repo: "deb [arch=arm64] https://download.docker.com/linux/{{ ansible_distribution | lower }} {{ ansible_distribution_release }} stable"
     update_cache: yes
+  when: ansible_architecture == "aarch64"
+
+- name: Add amd64 Docker apt repo to souces.list
+  apt_repository:
+    codename: "{{ ansible_distribution_release }}"
+    repo: "deb [arch=amd64] https://download.docker.com/linux/{{ ansible_distribution | lower }} {{ ansible_distribution_release }} stable"
+    update_cache: yes
+  when: ansible_architecture == "amd64"
 
 - name: Install Docker CE
   apt: