Docker的三大核心概念：镜像、容器、仓库
镜像：类似虚拟机的镜像、用俗话说就是安装文件。
容器：类似一个轻量级的沙箱，容器是从镜像创建应用运行实例，
可以将其启动、开始、停止、删除、而这些容器都是相互隔离、互不可见的。
仓库：类似代码仓库，是Docker集中存放镜像文件的场所。
简单介绍一下在CentOS上安装Docker。
前置条件：
64-bit 系统
kernel 3.10+
1.检查内核版本，返回的值大于3.10即可。
$ uname -r
2.使用 sudo 或 root 权限的用户登入终端。
3.确保yum是最新的
$ yum update
4.添加 yum 仓库
tee /etc/yum.repos.d/docker.repo <<-'EOF' [dockerrepo] name=Docker Repository baseurl=https://yum.dockerproject.org/repo/main/centos/$releasever/ enabled=1 gpgcheck=1 gpgkey=https://yum.dockerproject.org/gpg EOF
5.安装 Docker
$ yum install -y docker-engine
安装成功后，使用docker version命令查看是否安装成功，安装成功后------如下图
6.启动docker
$systemctl start docker.service
7.验证安装是否成功(有client和service两部分表示docker安装启动都成功了)
使用docker version命令查看
8.设置开机自启动
$ sudo systemctl enable docker
到此为止docker就完全安装好了。
CentOS7安装docker 启动不了解决篇
发现docker启动不了了，查看报错如下：
[root@ALY-HKC-PRO-001 ~]# systemctl restart docker
Job for docker.service failed because the control process exited with error code. See "systemctl status docker.service" and "journalctl -xe" for details.
[root@ALY-HKC-PRO-001 ~]# systemctl status docker
● docker.service - Docker Application Container Engine
Loaded: loaded (/usr/lib/systemd/system/docker.service; disabled; vendor preset: disabled)
Active: failed (Result: exit-code) since Mon 2017-07-03 10:12:16 CST; 5s ago
Docs: http://docs.docker.com
Process: 16172 ExecStart=/usr/bin/dockerd-current --add-runtime docker-runc=/usr/libexec/docker/docker-runc-current --default-runtime=docker-runc --exec-opt native.cgroupdriver=systemd --userland-proxy-path=/usr/libexec/docker/docker-proxy-current $OPTIONS $DOCKER_STORAGE_OPTIONS $DOCKER_NETWORK_OPTIONS $ADD_REGISTRY $BLOCK_REGISTRY $INSECURE_REGISTRY (code=exited, status=1/FAILURE)
Main PID: 16172 (code=exited, status=1/FAILURE)
Jul 03 10:12:15 ALY-HKC-PRO-001 systemd[1]: Starting Docker Application Container Engine...
Jul 03 10:12:15 ALY-HKC-PRO-001 dockerd-current[16172]: time="2017-07-03T10:12:15.812330534+08:00" level=info msg="libcontainerd: new containerd process, pid: 16180"
Jul 03 10:12:16 ALY-HKC-PRO-001 dockerd-current[16172]: time="2017-07-03T10:12:16.823237678+08:00" level=warning msg="devmapper: Usage of loopback devices is strongly discourage...v section."
Jul 03 10:12:16 ALY-HKC-PRO-001 dockerd-current[16172]: time="2017-07-03T10:12:16.838204058+08:00" level=error msg="[graphdriver] prior storage driver "devicemapper" failed: d...run failed"
Jul 03 10:12:16 ALY-HKC-PRO-001 dockerd-current[16172]: time="2017-07-03T10:12:16.838278116+08:00" level=fatal msg="Error starting daemon: error initializing graphdriver: devmap...run failed"
Jul 03 10:12:16 ALY-HKC-PRO-001 systemd[1]: docker.service: main process exited, code=exited, status=1/FAILURE
Jul 03 10:12:16 ALY-HKC-PRO-001 systemd[1]: Failed to start Docker Application Container Engine.
Jul 03 10:12:16 ALY-HKC-PRO-001 systemd[1]: Unit docker.service entered failed state.
Jul 03 10:12:16 ALY-HKC-PRO-001 systemd[1]: docker.service failed.
Hint: Some lines were ellipsized, use -l to show in full.
我们可以看到此处它提示是Failed to start Docker Application Container Engine.
于是在网上找各种类似错误，有说是需要把firewalld.service及selinux关闭。呃，这个的话是能够导致docker启动不了，可博主这块儿不是因为这个原因导致。
博主第一时间能想到的只有重装docker，可反复删除几次后仍为解决问题。（期间以为会是kubernetes当时设置有关，于是将kubernetes的所有软件也删除，问题同样是没有解决）。
最后突然想起很早的时候有看到文章说docker的engine 基于Device Mapper提供的一种存储驱动,而它又依赖与 devicemapper。于是乎我将它给删除并重启服务，解决了docker无法正常启动的问题。
rm -rf /var/lib/docker/
# 添加如下内容
vim /etc/docker/daemon.json
{
"graph": "/mnt/docker-data",
"storage-driver": "overlay"
}
[root@ALY-HKC-PRO-001 ~]# systemctl restart docker
[root@ALY-HKC-PRO-001 ~]# systemctl status docker
● docker.service - Docker Application Container Engine
Loaded: loaded (/usr/lib/systemd/system/docker.service; disabled; vendor preset: disabled)
Active: active (running) since Mon 2017-07-03 10:26:19 CST; 15s ago
Docs: http://docs.docker.com
Main PID: 17167 (dockerd-current)
CGroup: /system.slice/docker.service
├─17167 /usr/bin/dockerd-current --add-runtime docker-runc=/usr/libexec/docker/docker-runc-current --default-runtime=docker-runc --exec-opt native.cgroupdriver=systemd --userlan...
└─17175 /usr/bin/docker-containerd-current -l unix:///var/run/docker/libcontainerd/docker-containerd.sock --shim docker-containerd-shim --metrics-interval=0 --start-timeout 2m -...
Jul 03 10:26:18 ALY-HKC-PRO-001 dockerd-current[17167]: time="2017-07-03T10:26:18.961963315+08:00" level=info msg="Graph migration to content-addressability took 0.00 seconds"
Jul 03 10:26:18 ALY-HKC-PRO-001 dockerd-current[17167]: time="2017-07-03T10:26:18.962211996+08:00" level=warning msg="mountpoint for pids not found"
Jul 03 10:26:18 ALY-HKC-PRO-001 dockerd-current[17167]: time="2017-07-03T10:26:18.962703210+08:00" level=info msg="Loading containers: start."
Jul 03 10:26:19 ALY-HKC-PRO-001 dockerd-current[17167]: .........time="2017-07-03T10:26:19.230715822+08:00" level=info msg="Firewalld running: false"
Jul 03 10:26:19 ALY-HKC-PRO-001 dockerd-current[17167]: time="2017-07-03T10:26:19.379894482+08:00" level=info msg="Default bridge (docker0) is assigned with an IP address 172.30...IP address"
Jul 03 10:26:19 ALY-HKC-PRO-001 dockerd-current[17167]: time="2017-07-03T10:26:19.435937423+08:00" level=info msg="Loading containers: done."
Jul 03 10:26:19 ALY-HKC-PRO-001 dockerd-current[17167]: time="2017-07-03T10:26:19.436016788+08:00" level=info msg="Daemon has completed initialization"
Jul 03 10:26:19 ALY-HKC-PRO-001 dockerd-current[17167]: time="2017-07-03T10:26:19.436052792+08:00" level=info msg="Docker daemon" commit="1398f24/1.12.6" graphdriver=devicemappe...sion=1.12.6
Jul 03 10:26:19 ALY-HKC-PRO-001 dockerd-current[17167]: time="2017-07-03T10:26:19.441788942+08:00" level=info msg="API listen on /var/run/docker.sock"
Jul 03 10:26:19 ALY-HKC-PRO-001 systemd[1]: Started Docker Application Container Engine.
Hint: Some lines were ellipsized, use -l to show in full.
第二种方法作为参考：
[root@apptest ~]# yum remove docker
[root@apptest ~]# yum remove docker-selinux
[root@test yum.repos.d]# vi /etc/yum.repos.d/docker.repo
[dockerrepo]
name=Docker Repository
baseurl=https://yum.dockerproject.org/repo/main/centos/$releasever/
enabled=1
gpgcheck=1
[root@test yum.repos.d]# yum install docker-engine
已安装:
docker-engine.x86_64 0:1.10.2-1.el7.centos
作为依赖被安装:
docker-engine-selinux.noarch 0:1.10.2-1.el7.centos
[root@apptest ~]# service docker start
Redirecting to /bin/systemctl start docker.service
[root@apptest ~]# docker images
REPOSITORY TAG IMAGE ID CREATED SIZE
[root@test yum.repos.d]# docker version
Client:
Version: 1.10.2
API version: 1.22
Go version: go1.5.3
Git commit: c3959b1
Built: Mon Feb 22 16:16:33 2016
OS/Arch: linux/amd64
Server:
Version: 1.10.2
API version: 1.22
Go version: go1.5.3
Git commit: c3959b1
Built: Mon Feb 22 16:16:33 2016
OS/Arch: linux/amd64
[root@test yum.repos.d]#
[root@apptest ~]# yum remove docker
[root@apptest ~]# yum remove docker-selinux
[root@test yum.repos.d]# vi /etc/yum.repos.d/docker.repo
[dockerrepo]
name=Docker Repository
baseurl=https://yum.dockerproject.org/repo/main/centos/$releasever/
enabled=1
gpgcheck=1
[root@test yum.repos.d]# yum install docker-engine
已安装:
docker-engine.x86_64 0:1.10.2-1.el7.centos
作为依赖被安装:
docker-engine-selinux.noarch 0:1.10.2-1.el7.centos
[root@apptest ~]# service docker start
Redirecting to /bin/systemctl start docker.service
[root@apptest ~]# docker images
REPOSITORY TAG IMAGE ID CREATED SIZE
[root@test yum.repos.d]# docker version
Client:
Version: 1.10.2
API version: 1.22
Go version: go1.5.3
Git commit: c3959b1
Built: Mon Feb 22 16:16:33 2016
OS/Arch: linux/amd64
Server:
Version: 1.10.2
API version: 1.22
Go version: go1.5.3
Git commit: c3959b1
Built: Mon Feb 22 16:16:33 2016
OS/Arch: linux/amd64
[root@test yum.repos.d]#