container: Initialize container_networks to an empty list

roles/container/tasks/init.yml

@@ -7,6 +7,7 @@
     docker_volumes_new: []
     final_docker_volumes: "{{ docker_volumes }}"
     container_image: ''
+    container_networks: []
 
 - name: Warn about docker_volumes legacy format
   debug:

roles/container/tasks/main.yml

@@ -96,7 +96,7 @@
     command: "{{ docker_command | default(omit) }}"
     restart_policy: always
     network_mode: "{{ docker_network_mode | default(omit) }}"
-    networks: "{{ container_networks | default(omit) }}"
+    networks: "{{ container_networks }}"
     log_driver: local
   register: container_out