I think I made a big mistake.
I did usermod -aG docker $USER to run docker without sudo.
After run above command, my account status like below
joont@joont-ununtu:~$ id
uid=1000(joont) gid=1000(joont) groups=1000(joont),999(docker)
Now I can't use sudo command through this user....
I didn't set root password after installing ubuntu.
So I don't know my root password now.
How can I recover it..?