安装日志：
$ sudo apt install docker.io
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following additional packages will be installed:
bridge-utils cgroupfs-mount containerd git git-man liberror-perl pigz runc ubuntu-fan
Suggested packages:
aufs-tools btrfs-progs debootstrap docker-doc rinse zfs-fuse | zfsutils git-daemon-run | git-daemon-sysvinit git-doc git-el git-email git-gui gitk gitweb git-cvs git-mediawiki git-svn
The following NEW packages will be installed:
bridge-utils cgroupfs-mount containerd docker.io git git-man liberror-perl pigz runc ubuntu-fan
0 upgraded, 10 newly installed, 0 to remove and 384 not upgraded.
Need to get 56.9 MB of archives.
After this operation, 291 MB of additional disk space will be used.
Do you want to continue? [Y/n] y
Get:1 http://cn.archive.ubuntu.com/ubuntu bionic/universe amd64 pigz amd64 2.4-1 [57.4 kB]
Get:2 http://cn.archive.ubuntu.com/ubuntu bionic/main amd64 bridge-utils amd64 1.5-15ubuntu1 [30.1 kB]
Get:3 http://cn.archive.ubuntu.com/ubuntu bionic/universe amd64 cgroupfs-mount all 1.4 [6,320 B]
Get:4 http://cn.archive.ubuntu.com/ubuntu bionic-updates/universe amd64 runc amd64 1.0.0~rc7+git20190403.029124da-0ubuntu1~18.04.2 [1,903 kB]
Get:5 http://cn.archive.ubuntu.com/ubuntu bionic-updates/universe amd64 containerd amd64 1.2.6-0ubuntu1~18.04.2 [19.4 MB]
Get:6 http://cn.archive.ubuntu.com/ubuntu bionic-updates/universe amd64 docker.io amd64 18.09.7-0ubuntu1~18.04.3 [30.7 MB]
Get:7 http://cn.archive.ubuntu.com/ubuntu bionic/main amd64 liberror-perl all 0.17025-1 [22.8 kB]
Get:8 http://cn.archive.ubuntu.com/ubuntu bionic-updates/main amd64 git-man all 1:2.17.1-1ubuntu0.4 [803 kB]
Get:9 http://cn.archive.ubuntu.com/ubuntu bionic-updates/main amd64 git amd64 1:2.17.1-1ubuntu0.4 [3,907 kB]
Get:10 http://cn.archive.ubuntu.com/ubuntu bionic/main amd64 ubuntu-fan all 0.12.10 [34.7 kB]
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Setting up containerd (1.2.6-0ubuntu1~18.04.2) ...
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Setting up git (1:2.17.1-1ubuntu0.4) ...
Setting up docker.io (18.09.7-0ubuntu1~18.04.3) ...
Adding group `docker' (GID 127) ...
Done.
Created symlink /etc/systemd/system/sockets.target.wants/docker.socket → /lib/systemd/system/docker.socket.
Processing triggers for ureadahead (0.100.0-20) ...
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安装后，当前用户（普通用户）不能直接使用docker命令，需要更新。
$ docker ps Got permission denied while trying to connect to the Docker daemon socket at unix:///var/run/docker.sock: Get http://%2Fvar%2Frun%2Fdocker.sock/v1.39/containers/json:
dial unix /var/run/docker.sock: connect: permission denied
解决：添加当前用户到docker用户组
创建用户组 $ sudo groupadd docker 添加当前用户到docker组 $ sudo usermod -aG docker $USER 重启docker $ service docker restart
其实，重启docker后普通用户也直接使用docker命令，仍然提示没有权限。
此时，执行下面的命令可以解决问题：
$ newgrp - docker
Finally，重启Ubuntu解决了问题。
权限问题由下面的文件造成：
/var/run$ ls -l | grep docker drwx------ 5 root root 120 7月 17 14:35 docker -rw-r--r-- 1 root root 4 7月 17 14:35 docker.pid srw-rw---- 1 root docker 0 7月 17 14:35 docker.sock
Docker版本：
$ docker version
Client:
Version: 18.09.7
API version: 1.39
Go version: go1.10.1
Git commit: 2d0083d
Built: Wed Jul 3 12:13:59 2019
OS/Arch: linux/amd64
Experimental: false
Server:
Engine:
Version: 18.09.7
API version: 1.39 (minimum version 1.12)
Go version: go1.10.1
Git commit: 2d0083d
Built: Mon Jul 1 19:31:12 2019
OS/Arch: linux/amd64
Experimental: false
疑问：docker用户和docker的关系是什么？
修改为国内镜像：网易的、Docker中国的
daemon.json默认是没有的，需要新建，，
/etc/docker$ sudo vim daemon.json
/etc/docker$ cat daemon.json
{
"registry-mirrors": ["http://hub-mirror.c.163.com", "https://registry.docker-cn.com"]
}
安装完毕，接下来，使用各种docker命令实战。