Basics

Docker Basics

Docker is a containerization platform that allows developers to package applications and their dependencies into lightweight, portable containers. Containers ensure consistent behavior across different environments, from development to production.

What is Docker?

Docker enables you to run applications in isolated environments called containers. Unlike virtual machines, containers share the host OS kernel, making them more efficient and faster to start.

Key Benefits

Consistency: Applications run the same way everywhere
Isolation: Each container runs independently
Portability: Containers work across different platforms
Efficiency: Lower resource overhead than VMs

Core Concepts

Images vs Containers

Component	Description	Example
Image	Read-only template containing application code	`nginx:latest`
Container	Running instance of an image	Active web server
Dockerfile	Text file with instructions to build images	Recipe for creating images

Docker Architecture

graph TB A[Docker Client] --> B[Docker Daemon]; B --> C[Images]; B --> D[Containers]; B --> E[Networks]; B --> F[Volumes]; C --> G[Docker Registry/Hub];

Essential Commands

Image Management

docker pull <image> - Download an image
docker images - List local images
docker build -t <name> . - Build image from Dockerfile

Container Operations

docker run <image> - Create and start a new container
docker ps - List running containers
docker ps -a - List all containers (including stopped)
docker stop <container> - Stop a running container
docker rm <container> - Remove a container

Common Run Options

-d - Run in background (detached mode)
-p 8080:80 - Map host port 8080 to container port 80
-v /host/path:/container/path - Mount volume
--name myapp - Assign custom name

Docker Workflow

sequenceDiagram participant Dev as Developer participant DF as Dockerfile participant Img as Image participant Con as Container Dev->>DF: 1. Write Dockerfile Dev->>Img: 2. docker build Dev->>Con: 3. docker run Con->>Dev: 4. Application running

Dockerfile Basics

A Dockerfile contains step-by-step instructions to build an image:

FROM node:16
WORKDIR /app
COPY package.json .
RUN npm install
COPY . .
EXPOSE 3000
CMD ["npm", "start"]

Common Instructions

FROM - Base image to start from
WORKDIR - Set working directory
COPY - Copy files from host to container
RUN - Execute commands during build
EXPOSE - Document which ports the app uses
CMD - Default command when container starts

Best Practices

Security & Efficiency

Use official base images when possible
Keep images small by using minimal base images
Don't run containers as root user
Use .dockerignore to exclude unnecessary files

Development Workflow

Write application code
Create Dockerfile
Build image locally
Test container functionality
Push to registry for deployment

Docker fundamentally changes how applications are packaged and deployed, providing consistency and reliability across the entire software delivery pipeline.

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