Docker Container Basics

What is the difference between a Docker image and a container? How do they relate to each other?

Dockerbeginner

// interview question

What is the difference between a Docker image and a container? How do they relate to each other?

Sample answer

A Docker image is a read-only template containing application code, dependencies, and configuration. A container is a running instance of an image. You can create multiple containers from one image. Images are built from Dockerfiles and stored in registries. Containers are ephemeral - they can be started, stopped, and destroyed.

Why this matters

Understanding the image/container distinction is fundamental to Docker. Think of an image as a class definition and a container as an object instance. This relationship enables consistent deployments across environments - the same image produces identical containers everywhere.

Code examples

Working with images and containers

bash

# Pull an image from registry
docker pull nginx:latest

# List images
docker images

# Create and run a container from image
docker run -d --name my-nginx nginx:latest

# List running containers
docker ps

# List all containers (including stopped)
docker ps -a

Simple Dockerfile

dockerfile

FROM node:20-alpine
WORKDIR /app
COPY package*.json ./
RUN npm install
COPY . .
EXPOSE 3000
CMD ["node", "server.js"]

Common mistakes to avoid

Confusing images with containers
Not cleaning up stopped containers and unused images (disk space issues)
Running containers as root without considering security implications

Likely follow-ups

What happens to data inside a container when it stops?
How do you persist data using Docker volumes?
What is the difference between docker run and docker start?

Answer out loud first, then check yourself against the model answer.

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