What is a Docker Image?

A Docker Image is a read-only template used to create Docker containers. It contains the application code, runtime, libraries, environment variables, and configuration files needed to run an application.

Think of an image like a class in OOP — you define it once and create multiple containers (objects) from it.

Docker Image Commands

Pulling an Image from Docker Hub

# Pull latest nginx image
docker pull nginx

# Pull specific version
docker pull nginx:1.25

# Pull ubuntu image
docker pull ubuntu:22.04

Listing Local Images

docker images

Output:

REPOSITORY   TAG       IMAGE ID       CREATED        SIZE
nginx        latest    61395b4c586d   2 weeks ago    187MB
ubuntu       22.04     3b418d7b466a   4 weeks ago    77.8MB

Build an Image from Dockerfile

Create a simple Dockerfile:

FROM ubuntu:22.04
RUN apt-get update && apt-get install -y curl
CMD ["bash"]

Build the image:

# Build image with tag name myapp
docker build -t myapp .

Verify the image was created:

docker images

Tagging an Image

# Tag image for Docker Hub
docker tag myapp yourusername/myapp:v1.0

Removing an Image

# Remove by image name
docker rmi nginx

# Remove by image ID
docker rmi 61395b4c586d

# Force remove even if container is using it
docker rmi -f nginx

Image Layers

Docker images are built in layers. Each instruction in a Dockerfile creates a new layer. Layers are cached, so rebuilding is fast when only a few layers change.

# View image history and layers
docker history nginx

Summary

• A Docker image is a read-only blueprint for creating containers.
• Use docker pull to download images and docker build to create your own.
• Images are made of layers — cached for faster builds.
• Use docker rmi to remove unused images and free disk space.