Docker CEはDebian 9にサービスとして登録されていません。

リソースに応じてDocker Community Editionをインストールしました。https://linuxize.com/post/how-to-install-and-use-docker-on-debian-9/

ただし、ステップ5（systemctl status docker）に達すると、サービス状態の代わりに次のエラーメッセージが表示されます。

root@srv-012:/etc/systemd# systemctl start docker
Failed to start docker.service: Unknown unit: docker.service
See system logs and 'systemctl status docker.service' for details.

すべてのコマンドはrootとして実行されます。この問題をどのように解決できますか？

編集する：

The file `/lib/systemd/system/docker.service` is created correctly, here is the contents:

[Unit]
Description=Docker Application Container Engine
Documentation=https://docs.docker.com
BindsTo=containerd.service
After=network-online.target firewalld.service
Wants=network-online.target

[Service]
Type=notify
# the default is not to use systemd for cgroups because the delegate issues still
# exists and systemd currently does not support the cgroup feature set     required
# for containers run by docker
ExecStart=/usr/bin/dockerd -H unix://
ExecReload=/bin/kill -s HUP $MAINPID
TimeoutSec=0
RestartSec=2
Restart=always

# Note that StartLimit* options were moved from "Service" to "Unit" in     systemd 229.
# Both the old, and new location are accepted by systemd 229 and up, so using the old location
# to make them work for either version of systemd.
StartLimitBurst=3

# Note that StartLimitInterval was renamed to StartLimitIntervalSec in systemd 230.
# Both the old, and new name are accepted by systemd 230 and up, so using the old name to make
# this option work for either version of systemd.
StartLimitInterval=60s

# Having non-zero Limit*s causes performance problems due to accounting overhead
# in the kernel. We recommend using cgroups to do container-local accounting.
LimitNOFILE=infinity
LimitNPROC=infinity
LimitCORE=infinity

# Comment TasksMax if your systemd version does not supports it.
# Only systemd 226 and above support this option.
TasksMax=infinity

# set delegate yes so that systemd does not reset the cgroups of docker containers
Delegate=yes

# kill only the docker process, not all processes in the cgroup
KillMode=process

[Install]
WantedBy=multi-user.target