[docker] How to stop docker under Linux

My version of OS Ubuntu 16.04. I want to stop docker, so I run in the terminal:

sudo systemctl stop docker

But this commands doesn't help me:

gridsim1103 ~: ps ax | grep docker 
11347 ?        Sl     0:00 containerd-shim 487e3784f983274131d37bde1641db657e76e41bdd056f43ef4ad5adc1bfc518 /var/run/docker/libcontainerd/487e3784f983274131d37bde1641db657e76e41bdd056f43ef4ad5adc1bfc518 runc
14299 pts/2    S+     0:00 grep --color=auto docker
29914 ?        S      0:00 sudo dockerd -H gridsim1103:2376
29915 ?        Sl     4:45 dockerd -H gridsim1103:2376
29922 ?        Ssl    0:24 containerd -l unix:///var/run/docker/libcontainerd/docker-containerd.sock --shim containerd-shim --metrics-interval=0 --start-timeout 2m --state-dir /var/run/docker/libcontainerd/containerd --runtime runc
30107 ?        Sl     1:01 /usr/bin/docker-proxy -proto tcp -host-ip 188.184.80.77 -host-port 8500 -container-ip 192.17.0.2 -container-port 8500
30139 ?        Sl     0:00 /usr/bin/docker-proxy -proto tcp -host-ip 188.184.80.77 -host-port 8400 -container-ip 192.17.0.2 -container-port 8400

Version of docker server is:

Server:
 Version:      1.12.1
 API version:  1.24 (minimum version )
 Go version:   go1.6.2
 Git commit:   23cf638
 Built:        Tue, 27 Sep 2016 12:25:38 +1300
 OS/Arch:      linux/amd64
 Experimental: false

I also unsuccessfully tried:

 sudo service docker stop

In my case, it was neither systemd nor a cron job, but it was snap. So I had to run:

sudo snap stop docker
sudo snap remove docker

... and the last command actually never ended, I don't know why: this snap thing is really a pain. So I also ran:

sudo apt purge snap

:-)

if you have no systemctl and started the docker daemon by:

sudo service docker start

you can stop it by:

sudo service docker stop