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Aisalkyn Aidarova
Aisalkyn Aidarova

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Docker

#docker #containers #devops #beginners

*Why Do We Need Docker? *

In one of my previous projects, we had to set up a complete application that used:

  • NodeJS as the web server
  • MongoDB as the database
  • Redis as the caching/messaging system

Each part of the application needed different software, dependencies, versions, and configurations. This caused many problems:

Problems We Faced

  1. OS Compatibility Issues
    Some versions of MongoDB worked on certain operating systems, while Redis or NodeJS needed something else.

  2. Dependency Conflicts
    One service needed one version of a library, and another service needed a different version. They could not run together easily.

  3. Frequent Upgrades & Changes
    When we changed one service, we had to carefully check that everything still worked together. This took a lot of time.

  4. Developer Onboarding Was Hard
    A new developer had to:

  • Install the correct OS
  • Install the exact software versions
  • Follow long setup instructions If anything was different → the application would break.
  1. Different Environments Someone on Ubuntu, someone on Mac, someone on Windows — the app worked differently in every machine. So we could not guarantee the application would behave the same everywhere.

This whole problem is known as:

The “Matrix from Hell”

Managing compatibility between:

  • OS
  • Libraries
  • Versions
  • Tools
  • Dependencies

And doing it again every time something changes.

How Docker Solved This

With Docker:

  • Each service runs inside its own container
  • Each container has its own libraries and dependencies
  • But all containers share the same OS kernel

So:

  • No conflicts
  • No OS version problems
  • No manual setup
  • Just one command to start everything: 
docker run ...
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A developer only needs Docker installed, nothing else.

What Is a Container?

A container is:

  • An isolated environment
  • Runs its own processes
  • Has its own libraries and settings
  • But it shares the host OS kernel

Important Point

Containers are not virtual machines.

Operating System Basics (For Understanding Docker)

Every OS has two main parts:

Part Purpose
Kernel Talks to the hardware (same across Linux distros)
User Space Software UI, drivers, tools, libraries (this differs)

Ubuntu, Fedora, CentOS → all share the Linux kernel, but their user software differs.

What Docker Shares

Docker containers share the kernel of the host machine.

So:

  • If host OS = Linux → containers must also be Linux-based.
  • You cannot run Windows containers on Linux directly.

When you run Linux containers on Windows:

  • Windows actually runs a hidden Linux VM under the hood.

Containers vs Virtual Machines

Feature Virtual Machine Docker Container
OS Full OS per VM Shares host OS
Size GBs MBs
Startup Time Minutes Seconds
Isolation Very strong Medium
Resource Usage Heavy Light

Important:
It is not VMs vs Containers.
We use both together in real companies.

Example:

  • One big VM may run hundreds of containers.

Images vs Containers

Term Meaning
Image A package/template (like a VM template)
Container A running instance based on an image

We can start multiple containers from one image.

Docker in DevOps

Earlier:

  • Developers wrote code
  • Ops had to set it up manually → many errors

Now:

  • Dev + Ops create a Dockerfile together
  • Dockerfile → builds the image
  • The same image works everywhere (dev, test, prod)

This makes software:

  • Portable
  • Consistent
  • Easy to ship
  • Easy to scale

Conclusion

Docker solves:

  • Compatibility problems
  • Dependency conflicts
  • Slow onboarding
  • “Works on my machine” issues

It allows you to:

  • Package your app once
  • Run it anywhere

Docker comes in two editions:

Edition Description Cost
Docker Community Edition (CE) Free and open-source. Used for learning, personal projects, and most development work. Free
Docker Enterprise Edition (EE) Includes official support, security features, image and policy management, and enterprise orchestration tools. Paid / Licensed

For learning and real DevOps practice, Docker Community Edition (CE) is more than enough.

Where You Can Install Docker

Docker CE can be installed on:

  • Linux (Ubuntu, CentOS, Debian, etc.)
  • Mac (using Docker Desktop)
  • Windows (using Docker Desktop)
  • Cloud platforms (AWS, Azure, GCP virtual machines)

Installation Options

If You Are on Linux

You can directly install Docker CE using package manager:

apt   (Ubuntu / Debian)
yum  (RHEL / CentOS / Amazon Linux)
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This is what we will do in the demo.

If You Are on Mac or Windows

You have two choices:

Option 1 (Recommended for Learning):

Install a Linux Virtual Machine (VM) using:

  • VirtualBox
  • VMware
  • Hyper-V
  • Parallels (Mac)

Then install Docker inside the Linux VM.
This gives you the same environment as a real production server.

Option 2 (Faster Setup):

Install Docker Desktop (native application):

OS Download
Mac https://www.docker.com/products/docker-desktop
Windows https://www.docker.com/products/docker-desktop

Docker Desktop creates a lightweight Linux environment internally so that containers can run.

Before We Continue

If your machine is:

OS What To Do
Linux Continue with the upcoming demo.
Mac or Windows Install Docker Desktop or create a Linux VM. Then come back to the demo.

Demo: Installing and Getting Started with Docker on Linux (Ubuntu)

Step 1: Choose a Supported System

First, make sure you are working on a machine that supports Docker:

  • Physical machine, laptop, or virtual machine.
  • Running a 64-bit Linux OS (in this demo, we use Ubuntu).

You can check your Ubuntu version with:

cat /etc/os-release
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Step 2: Go to Docker Documentation

Open your browser and go to:

https://docs.docker.com
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Click Get Docker, which leads to the Docker Engine Community Edition installation page.

From the left menu:

  1. Select Linux
  2. Select Ubuntu

Step 3: Uninstall Any Older Docker Versions

Before installation, ensure no old Docker packages exist:

sudo apt-get remove docker docker-engine docker.io containerd runc
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If nothing is present, it will simply return with no changes.

Step 4: Install Docker (Using the Convenience Script)

There are two ways to install Docker:

  • Manual repository setup
  • Automated script (easier)

We will use the automated method.

Run:

curl -fsSL https://get.docker.com -o get-docker.sh
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Then execute the script:

sudo sh get-docker.sh
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This will:

  • Add Docker repository
  • Install Docker Engine
  • Configure dependencies
  • Start Docker service

Wait a few minutes for installation to complete.

Step 5: Verify Installation

Check the installed Docker version:

docker --version
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Example output:

Docker version 19.03.1, build abcdef01
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Step 6: Run Your First Container

Go to Docker Hub:

https://hub.docker.com
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Search for whalesay (a fun test image).

Or simply run:

sudo docker run docker/whalesay cowsay "Hello World"
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Docker will:

  1. Pull the docker/whalesay image from Docker Hub (if not already present)
  2. Create a container
  3. Run it, printing a whale saying “Hello World”

Output example:

_______________
< Hello World >
---------------
       \     
        \
         <°)))><
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Congratulations — Docker is working successfully.

https://docs.docker.com/install/linux/docker-ce/ubuntu/

Basic Docker Commands — Explanation

1. Run a Container 

docker run <image-name>
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Example:

docker run nginx
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  • If the image is not on your system, Docker will download it from Docker Hub.
  • If the image already exists, it will just start the container.

2. List Containers

Command Meaning
docker ps Show only running containers
docker ps -a Show all containers, including stopped/exited ones

Each container has:

  • A Container ID
  • An Image name
  • A Status
  • A Random auto-generated name (e.g., silly_sammet)

3. Stop a Running Container 

docker stop <container-id or container-name>
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Example:

docker stop silly_sammet
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After stopping, the container will show in:

docker ps -a
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with status Exited.

4. Remove a Stopped Container 

docker rm <container-id or name>
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This deletes the container completely.

Verify:

docker ps -a
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5. List Images 

docker images
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Shows:

  • Image name
  • Tag
  • Size

Example images:

nginx
redis
ubuntu
alpine
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6. Remove an Image 

docker rmi <image-name>
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Important:
You can only delete an image if no container is using it.
Stop and remove containers first.

7. Pull an Image (Download Only, Do Not Run) 

docker pull ubuntu
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This downloads the image, but does not create a container.

Why Some Containers Exit Immediately

Example:

docker run ubuntu
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This container exits right away. Why?

  • Containers run a single process.
  • Ubuntu image has no application running, so it stops immediately.

To keep it running, run a command:

docker run ubuntu sleep 5
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This runs sleep 5 inside the container → it runs for 5 seconds, then exits.

8. Execute a Command in a Running Container 

docker exec <container-name> <command>
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Example:

docker exec sleepy_container cat /etc/hosts
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This lets you run commands inside an existing running container.

9. Run Containers in Foreground vs Background

Foreground (Attached Mode) — default 

docker run kodekloud/simple-webapp
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You see the output and your terminal is busy until the container stops.

Press:

CTRL + C
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→ Stops the container.

Background (Detached Mode) — recommended for services 

docker run -d kodekloud/simple-webapp
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  • Container runs in the background.
  • You get your terminal back.

Check running containers:

docker ps
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Attach back to it:

docker attach <container-id>
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You only need the first few characters of the container ID.

Summary Table

Action Command Example
Run a container docker run nginx
Run container in background docker run -d nginx
List containers docker ps
List all containers docker ps -a
Stop container docker stop <name>
Remove container docker rm <name>
List images docker images
Remove image docker rmi <image>
Pull image only docker pull ubuntu
Run command inside container docker exec <name> cat /etc/hosts

1. Running Containers

We use:

docker run <image-name>
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Example: Run a CentOS container:

docker run centos
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  • If the image is not available locally, Docker will download it from Docker Hub.
  • If the image is already downloaded, it will be used directly.

Docker Hub

Official images can be pulled simply by using their name:

docker run ubuntu
docker run nginx
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For your own images stored under your account:

docker run <username>/<image-name>
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Example:

docker run mmsumshad/ansible-playable
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2. Why CentOS Exited Immediately?

Containers only live while a process is running inside them.
CentOS has no default process, so it exits right away.

To keep it alive, run a command such as bash and attach to the container:

docker run -it centos bash
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  • -i = interactive
  • -t = allocate a terminal

You are now inside the container.

Check OS version:

cat /etc/*release*
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Exit the container:

exit
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3. Running Containers in Background

Run a container and keep it alive with sleep, in detached mode:

docker run -d centos sleep 20
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  • This starts the container
  • Runs sleep 20
  • Container stays alive for 20 seconds, then exits

4. Listing Containers

Command Description
docker ps Shows only running containers
docker ps -a Shows all containers (running + exited)

Example:

docker ps
docker ps -a
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5. Stopping a Running Container 

docker stop <container-id or name>
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Example:

docker stop serene_pasteur
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6. Removing Containers

Remove stopped containers:

docker rm <container-id or name>
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You can remove multiple containers at once:

docker rm 345 e0a 773
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7. Listing Images 

docker images
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This shows:

  • Image name
  • Tag
  • Size

Example output might include:

ubuntu
centos
busybox
hello-world
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8. Removing Images 

docker rmi <image-name>
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Important: You must remove containers first before deleting their image.

Example:

docker rm <container>
docker rmi centos
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9. Pull an Image (Download Only) 

docker pull ubuntu
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This downloads the image, but does not run a container.

10. Execute Commands in a Running Container 

docker exec <container-id> <command>
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Example: View OS release file:

docker exec <container-id> cat /etc/*release*
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This runs the command inside the running container.

Summary of Commands

Action Command
Run a container docker run ubuntu
Run container in background docker run -d ubuntu sleep 100
Attach shell docker run -it ubuntu bash
List running containers docker ps
List all containers docker ps -a
Stop container docker stop <id>
Remove container docker rm <id>
List images docker images
Remove image docker rmi <image>
Pull image only docker pull ubuntu
Run command inside container docker exec <id> <command>

More docker run Options

We already know how to run a container, for example:

docker run redis
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This runs the latest version of the Redis image.

1. Image Tags (Versions)

Every Docker image can have multiple versions, called tags.

Example:

docker run redis:4.0
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This runs Redis version 4.0, instead of the latest.

If you don’t specify a tag:

docker run redis
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Docker automatically uses the:

:latest
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tag.

Where do we find available tags?

Go to:

https://hub.docker.com
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Search for:

redis
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You will see a list of tags/versions in the description.

Example Tag Table:

Tag Meaning
latest The newest version defined by the maintainers
5.0.5 Specific Redis version
4.0 Older Redis release

2. Interactive and Terminal Options (-i and -t)

Consider a program that asks for your name:

Enter your name:
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If we run it inside Docker normally:

docker run myapp
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You cannot type input because containers run non-interactively by default.

To allow typing input, we use:

-i   → keep STDIN open (interactive mode)
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Example:

docker run -i myapp
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But this still does not show a proper terminal prompt.

To fix that, we add:

-t   → allocate a pseudo-terminal (TTY)
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So the correct way is:

docker run -it myapp
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This allows:

  • Input to be typed
  • Prompt/output to display correctly

Summary of -i and -t

Option Meaning Purpose
-i Interactive STDIN Allows you to type input
-t Allocate TTY Displays a proper terminal prompt
-it Both combined The standard way to open a shell inside a container

Common example:

docker run -it ubuntu bash
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This opens a shell inside an Ubuntu container.

3. Port Mapping (Next Topic)

Many applications inside containers run on internal ports (e.g., web servers).

We need port mapping to access them from our host machine.

Command format:

docker run -p <host-port>:<container-port> <image>
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Example (we will explain this in detail next):

docker run -p 8080:80 nginx
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Quick Recap

Task Command Example
Run latest Redis docker run redis
Run Redis version 4.0 docker run redis:4.0
Run container interactively docker run -it ubuntu bash
Allow input in a CLI program docker run -it myapp
Map host port to container port docker run -p 8080:80 nginx

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