container-instancesはrequiredなんだね。そりゃまあそうか。
start-task [--cluster <value>] --container-instances <value> [--enable-ecs-managed-tags | --no-enable-ecs-managed-tags] [--enable-execute-command | --disable-execute-command] [--group <value>] [--network-configuration <value>] [--overrides <value>] [--propagate-tags <value>] [--reference-id <value>] [--started-by <value>] [--tags <value>] --task-definition <value> [--cli-input-json | --cli-input-yaml] [--generate-cli-skeleton <value>] [--debug] [--endpoint-url <value>] [--no-verify-ssl] [--no-paginate] [--output <value>] [--query <value>] [--profile <value>] [--region <value>] [--version <value>] [--color <value>] [--no-sign-request] [--ca-bundle <value>] [--cli-read-timeout <value>] [--cli-connect-timeout <value>] [--cli-binary-format <value>] [--no-cli-pager] [--cli-auto-prompt] [--no-cli-auto-prompt]
