CI: Pin docker installation to <7

Docker 7.0.0 introduced a pre build check for tag regex, that is most
probably the culprit - see [1] and [2].

Depends-On: https://review.opendev.org/c/openstack/kolla-ansible/+/903364

[1]: a9b5494fd0
[2]: https://github.com/docker/docker-py/issues/3195

Change-Id: I147c9715eb922abb6675a2786beaec85971718a8

tests/playbooks/pre.yml

@@ -43,7 +43,7 @@
       command: "{{ virtualenv_path }}/bin/python -m pip install {{ zuul.project.src_dir }}"
 
     - name: Install docker python library
-      command: "{{ virtualenv_path }}/bin/python -m pip install docker"
+      command: "{{ virtualenv_path }}/bin/python -m pip install 'docker<7'"
       when: container_engine == "docker"
 
     - name: Install podman python library