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Docker Introduction

Introduction

Why Docker?

Docker is a platform designed to simplify the development, deployment, and running of applications inside containers.

Key benefits:

  • Consistency across environments (dev, test, production).
  • Lightweight compared to virtual machines.
  • Faster startup times.
  • Portable: runs anywhere Docker is supported.
  • Scalable: easily orchestrated with tools like Docker Swarm or Kubernetes.

Operating System & Kernel Basics

  • An operating system consists of the kernel (core) and user-space programs.
  • The kernel manages resources such as CPU, memory, networking, and storage.
  • Containers share the host’s kernel, making them lightweight compared to virtual machines (which run a full OS).

Virtualization vs Containers

  • Virtual Machines (VMs): Require a full OS per instance and a hypervisor to manage them.
  • Containers: Share the host OS kernel while isolating processes and resources.

What is a Container?

A container is an isolated unit that packages application code with its dependencies.
It ensures the application runs the same way, regardless of the environment.

Docker Fundamentals

Docker History

  • Released in 2013 by DotCloud (later renamed Docker, Inc.).
  • Built on Linux container (LXC) technology.
  • Revolutionized DevOps practices by making containers accessible and easy to use.

Docker Engine

Docker Engine is the core component that runs and manages containers. It consists of:

  • Docker Daemon (dockerd): Background service that manages images, containers, networks, and volumes.
  • Docker CLI: Command-line tool to interact with the daemon.
  • REST API: Provides programmatic access.

Images & Containers

  • Image: A read-only template used to create containers. (e.g., ubuntu:20.04)
  • Container: A running instance of an image with its own writable layer.

Containers vs Virtual Machines

Feature Containers Virtual Machines
Startup time Seconds Minutes
OS per instance Shared host kernel Full OS
Resource usage Lightweight Heavy
Portability High Medium

Windows Containers

  • Windows supports containers with two modes:
    • Windows Server containers: Share the kernel with the host.
    • Hyper-V containers: Run with extra isolation using a lightweight VM.

Docker Versions & Ecosystem

Docker Editions

  • Docker CE (Community Edition): Free and open-source, widely used.
  • Docker EE (Enterprise Edition): Commercial, with support and enterprise features.
  • Docker Desktop: GUI for Windows/macOS with Docker Engine, CLI, and Compose bundled.

Docker Hub

  • Official registry for container images.
  • Users can push and pull images (docker push, docker pull).
  • Public and private repositories are available.

Docker CLI Basics

CLI Overview

Commonly used commands:

docker --version
docker ps -a
docker run hello-world
docker stop <container_id>
docker rm <container_id>
docker rmi <image_id>

Container Lifecycle

  1. Create → docker create
  2. Start → docker start
  3. Run → docker run (create + start)
  4. Pause/Unpause → docker pause / docker unpause
  5. Stop → docker stop
  6. Remove → docker rm

Docker’s Layered File System

  • Images are built in layers.
  • Each instruction in a Dockerfile adds a new layer.
  • Containers add a writable layer on top of the image.

Volumes & Data Persistence

  • Volumes: Persistent storage managed by Docker (docker volume create).
  • Bind mounts: Map a host directory into a container.
  • Anonymous volumes: Automatically created, removed when the container is deleted.

Empty vs Pre-populated Volume Behavior:

  • If a named volume is empty, Docker copies container data into it.
  • If it already has data, Docker does not overwrite it.

Networking & Logging

Networking

  • Bridge: Default network driver (isolated private network).
  • Host: Shares the host’s networking namespace.
  • Overlay: Used in multi-host setups (Swarm/Kubernetes).

Publishing Ports

docker run -d -p 8080:80 nginx

Maps port 80 in container → 8080 on host.

Logging & Monitoring

  • STDIN, STDOUT, STDERR: Container input/output streams.
  • docker logs : View container logs.
  • docker stats: Real-time CPU, memory, and network usage.

Resource Limits

docker run -d --memory="512m" --cpus="1.0" nginx

Environment Variables

docker run -e ENV=production nginx

Working with Images

Naming & Tagging

Format: repository:tag (default tag is latest).
Example:

docker pull ubuntu:20.04

Save & Load Images

docker save myapp:1.0 > myapp.tar
docker load < myapp.tar

Docker Commit

Create an image from a running container:

docker commit <container_id> myapp:custom

Private Registries

  • Host your own registry:
docker run -d -p 5000:5000 --name registry registry:2

References