Images and Containers

Two concepts sit at the heart of Docker: images and containers. Understanding the relationship between them is essential before you can use Docker effectively.

What is a Docker Image?

A Docker image is a read-only template that contains everything needed to run an application: the operating system base layer, runtime, libraries, application code, and default configuration. Think of an image as a recipe or a class definition — it describes what a container should look like, but it is not itself running.

Images are composed of layers. Each instruction in a Dockerfile adds a new layer on top of the previous one. Layers are cached and shared between images, which is why downloading a second image that shares a base layer with one you already have is much faster than downloading the first.

# List images you have locally
docker images

# Pull an image without running it
docker pull nginx:latest

# Inspect image layers
docker history nginx:latest

# Remove an image
docker rmi nginx:latest

What is a Docker Container?

A container is a running instance of an image. When you run an image Docker creates a thin writable layer on top of the read-only image layers and starts the process defined in the image. Multiple containers can be started from the same image simultaneously; each gets its own writable layer and its own isolated process space.

The relationship is analogous to a class and its instances in object-oriented programming: the image is the class, containers are the instances.

Running Containers

# Run a container interactively and remove it when it exits
docker run --rm -it ubuntu:22.04 bash

# Run a container in the background (detached)
docker run -d --name my-nginx -p 8080:80 nginx

# List running containers
docker ps

# List all containers including stopped
docker ps -a

# Stop a running container
docker stop my-nginx

# Remove a stopped container
docker rm my-nginx

# Stop and remove in one command
docker rm -f my-nginx

Container Lifecycle

A container moves through several states:

• Created — the container exists but has not been started
• Running — the container's main process is executing
• Paused — the container's processes are suspended
• Stopped/Exited — the main process has terminated
• Removed — the container has been deleted

# Create without starting
docker create --name my-container nginx