Docker Container Basics

Question

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

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

Simple Dockerfile

dockerfile

Common Mistakes

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

Follow-up Questions

Interviewers often ask these as follow-up questions

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?