Ansible: Variable in a dictionary key

---
## Expected usage:
-
  name: "{{ website.fancy_name }} - nginx container"
  docker_container:
    ...
    labels:
      "traefik.enable": "true",
      "traefik.http.routers.{{ website.name }}.rule": "Host({{ nginx_traefik_host }})"



### Actual usage:
-
  name: "{{ website.fancy_name }} - nginx container"
  vars:
    # This hack is needed to allow using a variable in the Dictionary key.
    # @see https://github.com/ansible/ansible/issues/17324#issuecomment-498686519
    container_labels: |
        {
          "traefik.enable": "true",
          "traefik.http.routers.{{ website.name }}.rule": "Host({{ nginx_traefik_host }})"
        }
  docker_container:
    ...
    labels: "{{ container_labels }}"