centos 安装docker

看官网安装步骤，比所有博主写的好，错了还可以排错

https://docs.docker.com/install/linux/docker-ce/centos/#install-from-a-package

SET UP THE REPOSITORY

Install required packages. yum-utils provides the yum-config-manager utility, and device-mapper-persistent-data and lvm2 are required by the devicemapper storage driver.
```
$ sudo yum install -y yum-utils 
  device-mapper-persistent-data 
  lvm2
```
Use the following command to set up the stable repository. You always need the stable repository, even if you want to install builds from the edge or test repositories as well.
```
$ sudo yum-config-manager 
    --add-repo 
    https://download.docker.com/linux/centos/docker-ce.repo
```
Optional: Enable the edge and test repositories. These repositories are included in the docker.repo file above but are disabled by default. You can enable them alongside the stable repository.
```
$ sudo yum-config-manager --enable docker-ce-edge
```
```
$ sudo yum-config-manager --enable docker-ce-test
```
You can disable the edge or test repository by running the yum-config-manager command with the --disable flag. To re-enable it, use the --enable flag. The following command disables the edge repository.
```
$ sudo yum-config-manager --disable docker-ce-edge
```
Note: Starting with Docker 17.06, stable releases are also pushed to the edge and test repositories.

Learn about stable and edge builds.

INSTALL DOCKER CE

Install the latest version of Docker CE, or go to the next step to install a specific version.
```
$ sudo yum install docker-ce
```
Warning: If you have multiple Docker repositories enabled, installing or updating without specifying a version in the yum install or yum update command always installs the highest possible version, which may not be appropriate for your stability needs.

If this is the first time you are installing a package from a recently added repository, you are prompted to accept the GPG key, and the key’s fingerprint is shown. Verify that the fingerprint is correct, and if so, accept the key. The fingerprint should match 060A 61C5 1B55 8A7F 742B 77AA C52F EB6B 621E 9F35.

Docker is installed but not started. The docker group is created, but no users are added to the group.
On production systems, you should install a specific version of Docker CE instead of always using the latest. List the available versions. This example uses the sort -r command to sort the results by version number, highest to lowest, and is truncated.
```
$ yum list docker-ce --showduplicates | sort -r

docker-ce.x86_64            17.12.ce-1.el7.centos             docker-ce-stable
```
The contents of the list depend upon which repositories are enabled, and are specific to your version of CentOS (indicated by the .el7 suffix on the version, in this example). Choose a specific version to install. The second column is the version string. You can use the entire version string, but you need to include at least to the first hyphen. The third column is the repository name, which indicates which repository the package is from and by extension its stability level. To install a specific version, append the version string to the package name and separate them by a hyphen (-).

Note: The version string is the package name plus the version up to the first hyphen. In the example above, the fully qualified package name is docker-ce-17.06.1.ce.
```
$ sudo yum install <FULLY-QUALIFIED-PACKAGE-NAME>
```
Start Docker.
```
$ sudo systemctl start docker
```
Verify that docker is installed correctly by running the hello-world image.
```
$ sudo docker run hello-world
```
This command downloads a test image and runs it in a container. When the container runs, it prints an informational message and exits.

Docker CE is installed and running. You need to use sudo to run Docker commands. Continue to Linux postinstall to allow non-privileged users to run Docker commands and for other optional configuration steps.

UPGRADE DOCKER CE

To upgrade Docker CE, follow the installation instructions, choosing the new version you want to install.

Install from a package

If you cannot use Docker’s repository to install Docker, you can download the .rpm file for your release and install it manually. You need to download a new file each time you want to upgrade Docker.

Go to https://download.docker.com/linux/centos/7/x86_64/stable/Packages/ and download the .rpm file for the Docker version you want to install.

Note: To install an edge package, change the word stable in the above URL to edge. Learn about stable and edgechannels.
Install Docker CE, changing the path below to the path where you downloaded the Docker package.
```
$ sudo yum install /path/to/package.rpm
```
Docker is installed but not started. The docker group is created, but no users are added to the group.
Start Docker.
```
$ sudo systemctl start docker
```
Verify that docker is installed correctly by running the hello-world image.
```
$ sudo docker run hello-world
```
This command downloads a test image and runs it in a container. When the container runs, it prints an informational message and exits.

Docker CE is installed and running. You need to use sudo to run Docker commands. Continue to Post-installation steps for Linuxto allow non-privileged users to run Docker commands and for other optional configuration steps.

UPGRADE DOCKER CE

To upgrade Docker CE, download the newer package file and repeat the installation procedure, using yum -y upgrade instead of yum -y install, and pointing to the new file.