Docker+openspeedtest on 21.02 on Raspi4


I recently installed Openwrt 21.02.0-rc2 on Raspberry Pi 4B (4GB), connected it with a switch, it's working well. So I wanted to install openspeedtest. On the webpage it says
"install Docker and run
docker run --restart=unless-stopped --name=openspeedtest -d -p 80:8080 openspeedtest/latest "
so I did so. Then I got an error message:
docker: Cannot connect to the Docker daemon at unix:///var/run/docker.sock. Is the docker daemon running?.

See 'docker run --help'.

I saw docker run --help but still don't know what to do, or don't know what's about unix:///var/run/docker.sock
I searched for info but didn't find anything....
I appreciate if someone could tell me what to do !