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

问题

ubuntu安装好docker，执行了命令“sudo docker run hello-world”之后报错：
docker: Cannot connect to the Docker daemon at unix:///var/run/docker.sock. Is the docker daemon running?.

user@DESKTOP-QB5ALJE:~$ sudo docker run hello-world
docker: Cannot connect to the Docker daemon at unix:///var/run/docker.sock. Is the docker daemon running?.

解决办法：

先运行下面的命令：sudo service docker start * Starting Docker: docker
再运行：sudo docker run hello-world
成功解决~

user@DESKTOP-QB5ALJE:~$ sudo service docker start  * Starting Docker: docker
 * Starting Docker: docker                                                                                                                                                            [ OK ]
user@DESKTOP-QB5ALJE:~$ sudo docker run hello-world
Unable to find image 'hello-world:latest' locally
latest: Pulling from library/hello-world
2db29710123e: Pull complete
Digest: sha256:bfea6278a0a267fad2634554f4f0c6f31981eea41c553fdf5a83e95a41d40c38
Status: Downloaded newer image for hello-world:latest

Hello from Docker!
This message shows that your installation appears to be working correctly.

To generate this message, Docker took the following steps:
 1. The Docker client contacted the Docker daemon.
 2. The Docker daemon pulled the "hello-world" image from the Docker Hub.
    (amd64)
 3. The Docker daemon created a new container from that image which runs the
    executable that produces the output you are currently reading.
 4. The Docker daemon streamed that output to the Docker client, which sent it
    to your terminal.

To try something more ambitious, you can run an Ubuntu container with:
 $ docker run -it ubuntu bash

Share images, automate workflows, and more with a free Docker ID:
 https://hub.docker.com/

For more examples and ideas, visit:
 https://docs.docker.com/get-started/

• 参考资料：
  https://stackoverflow.com/questions/44678725/cannot-connect-to-the-docker-daemon-at-unix-var-run-docker-sock-is-the-docker