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Docker Introduction

Docker is a container runtime. It runs applications in isolated environments called containers.

Docker solves a simple problem. Apps break when environments differ.

Different machines have different:

  • OS packages
  • library versions
  • configs
  • runtime paths

Docker packages the app and its dependencies into an image. You run the image as a container. You get the same runtime behavior across machines.

Docker vs Virtual Machines​

A VM includes a full guest OS. A container shares the host OS kernel.

VMs:

  • boot a full OS
  • are heavier
  • start slower

Containers:

  • run as isolated processes
  • are lighter
  • start fast

Diagram:

Virtual Machines
----------------
Hardware
└─ Host OS
   └─ Hypervisor
      ├─ VM1 (Guest OS + App)
      ├─ VM2 (Guest OS + App)
      └─ VM3 (Guest OS + App)

Containers
----------
Hardware
└─ Host OS (Kernel)
   └─ Container Runtime (Docker)
      ├─ Container1 (App + libs)
      ├─ Container2 (App + libs)
      └─ Container3 (App + libs)

Containers are not “mini VMs”. They are processes with isolation.

Image vs Container​

Image:

  • read-only template
  • built once
  • shared across machines

Container:

  • running instance of an image
  • has a small writable layer
  • can be started/stopped/deleted

Diagram:

Image (read-only)
└─ Layers: base OS + runtime + app

Container (running)
└─ Image layers (read-only)
└─ Writable layer (container changes)

Key point:

  • You build images.
  • You run containers.

Docker vs Kubernetes​

Docker runs containers on one machine. Kubernetes runs containers across many machines.

Docker answers:

  • how to build an image
  • how to run a container

Kubernetes answers:

  • where to run it
  • how many to run
  • how to restart/scale
  • how to do rolling updates

Learn Docker first. Then Kubernetes becomes easier.

Docker’s main moving parts​

  • Docker Engine: the background service that runs containers
  • Docker CLI: the docker command you type
  • Registry: where images are stored (Docker Hub, ECR, ACR, etc.)

Diagram:

You (CLI)  ->  Docker Engine  ->  Images/Containers
                      |
                      └-> Registry (pull/push images)

Quick sanity checks​

These commands confirm Docker is installed and reachable.

docker version

Sample output (short):

Client: Docker Engine
 Version: 27.0.0

Server: Docker Engine
 Version: 27.0.0
docker info

Sample output (short):

Server Version: 27.0.0
Containers: 0
Images: 0
Storage Driver: overlay2

If docker version shows both Client and Server, you are ready for hands-on.