How to Install Docker on CentOS 7 Linux systems

In this article, I’ll show you how to install Docker in CentOS 7 Linux system. This is your first step to get started with Docker engine on CentOS 7. I’ll use a minimal installation of CentOS 7 64bit to install Docker on it, of course the following installation steps are valid for all CentOS 7 installation types “Server without  Gui, Server with Gui, minimal, etc..”.
On this machine we will install Docker engine, allowing you to run a docker image (container) within docker engine.

Introduction to Docker.

Docker is an open-source tool that makes creating & managing Linux containers(LXC) easy. Containers are like lightweight VMs which can be started & stopped in milliseconds. Dockers help the system administrators & coders to develop their application in a container and can further scale up to 1000 of nodes. Working with Docker containers adds many benefits for developers: better separation of application and the server configuration.

The main difference between container and VM(Virtual machine) is that dockers provide process based isolation , whereas VM provides full isolation of resources. Virtual machine takes a minute to start where as container can be started in a second or less than a second. Container uses the Kernel of host OS , whereas VM uses the separate Kernel. Container uses less disk space and memory than VM.

Prerequisites

Docker requires a 64-bit OS and version 3.10 or higher of the Linux kernel. So, we need to check these two requirements against our CentOS 7 server.

To check your current kernel version and OS architecture, run the following command:

# uname -r
3.18.44-20.el7.x86_64

Perfect, our server is has the prerequisites for installing Docker. Let’s start our steps for installing Docker.

# yum -y update

# tee /etc/yum.repos.d/docker.repo <<-'EOF'
 [dockerrepo]
 name=Docker Repository
 baseurl=https://yum.dockerproject.org/repo/main/centos/7/
 enabled=1
 gpgcheck=1
 gpgkey=https://yum.dockerproject.org/gpg
 EOF

# vim /etc/yum.repos.d/docker.repo

[dockerrepo]
name=Docker Repository
baseurl=https://yum.dockerproject.org/repo/main/centos/$releasever/
enabled=1
gpgcheck=1
gpgkey=https://yum.dockerproject.org/gpg

# yum repolist
Loaded plugins: fastestmirror, langpacks
Loading mirror speeds from cached hostfile
 * base: mirror.airenetworks.es
 * epel: mirrors.ircam.fr
 * extras: mirror.airenetworks.es
 * updates: mirror.airenetworks.es
repo id repo name status
!base/7/x86_64 CentOS-7 - Base 9,353+10
!dockerrepo Docker Repository 48+32
*!epel/x86_64 Extra Packages for Enterprise Linux 7 - x86_64 11,019
!extras/7/x86_64 CentOS-7 - Extras 435
!updates/7/x86_64 CentOS-7 - Updates 423+10
repolist: 24,594

As you see, Docker Repository exists.

# yum -y install docker-engine

# systemctl enable docker
# systemctl start docker

Hints: 

1. If you want normal user to use docker, you need to add this normal user to the docker group.
2. By adding a user to the docker group, you'll have a full privileges on different docker commands, See the following optional step.

Step 5: Running Docker with Normal User Privileges “Optional”

You need to add your normal user to the docker group created by default when we installed the Docker packages. Here’s I’ll add user “msemari” to docker group:

# usermod -aG docker msemari

Now, logout and login again using this user “your normal user” and enjoy playing with Docker.

$ docker run hello-world

Unable to find image 'hello-world:latest' locally
latest: Pulling from library/hello-world

4276590986f6: Pull complete
a3ed95caeb02: Pull complete
Digest: sha256:4f32210e234b4ad5cac92efacc0a3d602b02476c754f13d517e1ada048e5a8ba
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.
 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 Hub account:
 https://hub.docker.com

For more examples and ideas, visit:
 https://docs.docker.com/engine/userguide/

Extra Step: Removing Docker from your System

1. List the installed Docker packages.

   # yum list installed | grep docker
   
   docker-engine.x86_64                   1.12.3-1.el7.centos             @dockerrepo
   docker-engine-selinux.noarch           1.12.3-1.el7.centos             @dockerrepo
   

2. Remove the package.

   # yum -y remove docker-engine.x86_64
   # yum -y remove docker-engine-selinux.noarch
   

   This command does not remove images, containers, volumes, or user-created configuration files on your host.

3. To delete all images, containers, and volumes, run the following command:

   # rm -rf /var/lib/docker
   

4. Locate and delete any user-created configuration files.

Summary

I think, Docker if the future replacement of the VMs as it’s a lightweight server, it do not need a dedicated memory or disk or CPU power. Your server can run number of containers higher than the number of VMs with same server’s hardware. Personally, I’m moving from using VMs to use Docker containers to save my machines resources. Finally, with help of this article you could install Docker engine and enable, start it. I also included how to completely remove Docker packages.