ERROR: Couldn’t connect to Docker daemon at http+docker://localunixsocket - is it running?

ERROR: Couldn’t connect to Docker daemon at http+docker://localunixsocket - is it running?
docker-compose up-dタイムズエラーを実行します.

$ docker-compose up
ERROR: Couldn't connect to Docker daemon at http+docker://localhost - is it running?

If it's at a non-standard location, specify the URL with the DOCKER_HOST environment variable.

解決策:1、dockerを起動する:

          systemctl start docker
         sudo systemctl daemon-reload
  docker     systemctl restart  docker
  docker    sudo service docker restart
  docker   service docker stop   
  docker  systemctl stop docker

2、現在のユーザーをdockerユーザーグループに加入する:

$ sudo gpasswd -a ${USER} docker