7. Docker Images Explained: Layers, Building, Pulling & Managing Images

Docker – Images (Comprehensive Guide)

Docker Images are the foundation of containerization. A Docker Image is a read-only template that contains everything an application needs to run: source code, system libraries, dependencies, runtimes, configuration files, and environment instructions. When you run a Docker container, it is created from a Docker Image. Therefore, understanding images is essential for working effectively with Docker.

1. Understanding Docker Images

A Docker Image is built in a layered architecture. Each layer represents a change or instruction—for example installing software, copying files, or setting environment variables. Layers are stored efficiently, which means:

• Common layers are shared between images to save disk space.
• Rebuilding is fast because only changed layers are updated.
• Deployment is efficient because only missing layers are downloaded.

2. Where Do Docker Images Come From?

There are three main sources of Docker Images:

• Official Images from Docker Hub (e.g., nginx, mysql, node)
• Community Images shared by developers
• Custom-built Images using your own Dockerfile

You can browse images at: hub.docker.com

3. Pulling Images from Docker Hub

To download an image:

docker pull ubuntu

To download a specific version (using tags):

docker pull ubuntu:22.04

Every image should have a tag. If not specified, Docker uses the default tag latest.

4. Listing and Inspecting Images

To view all downloaded images:

docker images

To inspect details like layers, commands, and metadata:

docker inspect ubuntu

5. Creating Your Own Docker Image (Using Dockerfile)

A Dockerfile contains the instructions for building a custom image. Example:

FROM node:18
WORKDIR /app
COPY package*.json ./
RUN npm install
COPY . .
EXPOSE 3000
CMD ["npm", "start"]

Build this image:

docker build -t mynodeapp:v1 .

Run the container:

docker run -p 3000:3000 mynodeapp:v1

6. Tagging & Pushing Images (For Sharing or Deployment)

Tag the image to your Docker Hub username:

docker tag mynodeapp:v1 username/mynodeapp:v1

Push to Docker Hub:

docker push username/mynodeapp:v1

7. Removing Images

docker rmi image_name

8. Best Practices for Docker Images

• Use official base images for security & reliability.
• Use Alpine images to reduce image size (e.g., node:18-alpine).
• Always .dockerignore unnecessary files (node_modules, logs, build caches).
• Use descriptive tags like v1, v2, production, staging.
• Regularly scan images for vulnerabilities.

In summary, Docker Images make applications portable, consistent, and scalable across development, testing, staging, and production. Mastering Docker Images is the first essential step toward mastering containerization.