fix: docker external network declaration

[guillaumemichel]

External networks in docker compose should be declared as follow (source):

networks:
  default:
    name: $DEFAULT_NETWORK
    external: true

rather than

networks:
  default:
    external:
      name: $DEFAULT_NETWORK

Current configs trigger the warning:

WARN[0000] network default: network.external.name is deprecated. Please set network.name with external: true