get Docker error each time I reboot my machine: "Cannot connect to the Docker daemon at unix:///var/run/docker.sock."

I’m running on ubuntu 18.04 with docker version:

 >> docker version
Client: Docker Engine - Community
 Version:           19.03.6
 API version:       1.40
 Go version:        go1.12.16
 Git commit:        369ce74a3c
 Built:             Thu Feb 13 01:27:48 2020
 OS/Arch:           linux/amd64
 Experimental:      false

each time I reboot my machine and try to use docker (e.g docker ps), I get an error:

>> docker ps   
Cannot connect to the Docker daemon at unix:///var/run/docker.sock. Is the docker daemon running?

after I run:

>> service docker stop
>> service docker start

everything is back to normal..

any ideas how to solve it?

Source: Docker Questions