How to Install Docker on Debian 9 “Stretch” Linux systems

Posted by: Mohammed Semari | Published: December 9, 2018| Updated: December 9, 2018

In this article, I’ll show you how to install Docker on Debian 9 Linux system. This is your first step to get started with Docker engine on Debian 9. I’ll use a minimal installation of Debian 9 “Stretch” 64bit to install Docker on it, of course the following installation steps are valid for all Debian 9 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 Debian 9 server.

Hints:

1. You must be the root user to run the following steps and do not use sudo command. 
2. If you used sudo command, some of the following commands will give errors.

To be a root user, run the following command:

$ sudo su -

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

# uname -r
4.9.0-3-amd64

Perfect, our server is has the prerequisites for installing Docker. Let’s start our steps for installing Docker “You Must be root”.

Step 1: Adding the Docker Repository

For adding the Apt repository for Docker, we need to make sure that the following points are successfully ran on the server. These points for removing old Docker packages “if exists” and for adding the new APT repository fro Docker, run the following commands AS ROOT:

Purge any older repositories.

 # apt-get purge "lxc-docker*"
 # apt-get purge "docker.io*"

Update package information, ensure that APT works with the https method, and that CA certificates are installed.
```
 # apt-get update
 # apt-get -y install apt-transport-https ca-certificates gnupg2
```

Add the new GPG key.

 # apt-key adv \
       --keyserver hkp://ha.pool.sks-keyservers.net:80 \
       --recv-keys 58118E89F3A912897C070ADBF76221572C52609D

Create the docker.list file in /etc/apt/sources.list.d/ directory in your terminal run the following command:.
```
# echo "deb https://apt.dockerproject.org/repo debian-stretch main" > /etc/apt/sources.list.d/docker.list
```
Update the APT package index.
```
# apt-get update
```
Verify that APT is pulling from the right repository.
```
# apt-cache policy docker-engine
```
From now on when you run apt-get upgrade, APT pulls from the new apt repository.

Step 2: Installing the Docker engine

Before installing Docker, make sure you have set your APT repository correctly as described in the prerequisites. Run the following command to update the APT package index:

 # apt-get update

We can install docker-engine with the following command:

# apt-get -y install docker-engine

Perfect!, We installed the docker engine on Debian 9 Linux system.

Step 3: Enable and Start the Docker Service

The Docker engine is now installed on Debian 9, we need to enable it to run at boot and start the service, run the following commands:

# systemctl enable docker
# systemctl restart 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 4: 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.

Step 5: Deploying your First Docker Image, “Hello World!”

To be sure that we successfully installed Docker, we can start the all time classic “hello-world” image, Here’s I’ll run the following command with a normal user “the one added above”:

$ docker run hello-world

You will get the following output:

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/

Hooray! We just pulled our first image from Docker hub.

Extra Steps: Upgrading and Removing Docker from your System

Here’s I give you two extra points for upgrading your Docker engine to the latest version and to completely remove Docker engine from your Stretch Linux.

Upgrade Docker

To install the latest version of Docker with apt-get:

# apt-get upgrade docker-engine

Uninstall

To only uninstall the Docker package:

# apt-get purge docker-engine

To uninstall the Docker package and dependencies that are no longer needed:

# apt-get autoremove --purge docker-engine

The above commands will not remove images, containers, volumes, or user created configuration files on your host. If you wish to delete all images, containers, and volumes run the following command:

# rm -rf /var/lib/docker

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.

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