Add note about installilng 'docker' Python package if Docker installation is disabled

roles/matrix-base/defaults/main.yml

@@ -85,7 +85,7 @@ matrix_vars_yml_snapshotting_src: "{{ inventory_dir }}/host_vars/{{ inventory_ho
 matrix_well_known_matrix_server_enabled: true
 
 # Controls whether Docker is automatically installed.
-# If you change this to false you must install and update Docker manually.
+# If you change this to false you must install and update Docker manually. You also need to install the [`docker`](https://pypi.org/project/docker/) Python package.
 matrix_docker_installation_enabled: true
 
 # Controls the Docker package that is installed.