/usr/lib/systemd/system/docker.service

docker.service

[Unit]
Description=Docker Application Container Engine
Documentation=https://docs.docker.com
After=network.target docker.socket
Requires=docker.socket

[Service]
Type=notify
EnvironmentFile=-/etc/default/docker
# 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
ExecStartPre=/bin/sh -x -c "if [ ! -s /var/lib/docker/repositories-overlay ]; then rm -f /var/lib/docker/repositories-overlay; fi"
ExecStart=/usr/bin/docker daemon -s overlay --registry-mirror=https://mirror.gcr.io --host=fd:// $DOCKER_OPTS
MountFlags=slave
LimitNOFILE=1048576
LimitNPROC=1048576
LimitCORE=infinity
TimeoutStartSec=0
# set delegate yes so that systemd does not reset the cgroups of docker containers
Delegate=yes

[Install]
WantedBy=multi-user.target

https://cloud.google.com/container-optimized-os/docs/how-to/run-container-instance#starting_a_docker_container_via_cloud-config

May i know use of this file...

actually I don't remember why I made it.
seems I was trying to setup docker daemon on a cloud instance, and this was a memo of the config inside Container Optimized Machine of Google Cloud.

not sure what you're trying but that's all I can say now 😅