[train-only] install docker.com gpg key

The docker.com gpg key is required so we can install the containerd.io
rpm.

It has become a requirement in Ansible 2.9:
https://docs.ansible.com/ansible/latest/porting_guides/porting_guide_2.9.html#modules

Change-Id: Icb0133c7e14370237f5e94bcb0adf7d094cd0782
Closes-Bug: #1898037

zuul.d/playbooks/pre.yml

@@ -51,13 +51,19 @@
 
     # NOTE(cloudnull): This is being done because docker is not supported on RHEL-8
     #                  and tests within this repo still require docker.
-    - name: Manual install container.io
+    - name: Manual install of containerd.io from docker.com
       become: true
-      package:
-        name: https://download.docker.com/linux/centos/7/x86_64/stable/Packages/containerd.io-1.2.6-3.3.el7.x86_64.rpm
       when:
         - (ansible_os_family | lower) == "redhat"
         - (ansible_distribution_major_version | int) >= 8
+      block:
+        - name: Import GPG key from docker.com
+          rpm_key:
+            state: present
+            key: https://download.docker.com/linux/centos/gpg
+        - name: Install containerd.io
+          package:
+            name: https://download.docker.com/linux/centos/7/x86_64/stable/Packages/containerd.io-1.2.6-3.3.el7.x86_64.rpm
   tasks:
     - name: Get Ansible Galaxy roles
       command: >-