Why image optimization matters in Docker - Why It Works

Introduction

Large Docker images take longer to download and use more storage. Optimizing images makes your apps start faster and saves space on servers.

When you want your app to start quickly after deployment

When you have limited storage space on your servers or cloud

When you need to transfer images over slow or limited internet connections

When you want to reduce costs by using less bandwidth and storage

When you want to keep your development and production environments clean and efficient

Commands

This command lists all Docker images on your system with their sizes so you can see which images are large.

Terminal

docker images

Expected Output

REPOSITORY          TAG       IMAGE ID       CREATED        SIZE

nginx               latest    4bb46517cac3   2 weeks ago    133MB

my-app              v1        7a8f9d2c3b1a   3 days ago     850MB

Builds a Docker image named 'my-app:optimized' using the Dockerfile in the current folder, applying optimization techniques like smaller base images.

Terminal

docker build -t my-app:optimized .

Expected Output

Sending build context to Docker daemon  12.3MB

Step 1/5 : FROM alpine:3.18

 ---> a24bb4013296

Step 2/5 : COPY . /app

 ---> Using cache

Step 3/5 : RUN apk add --no-cache python3

 ---> Using cache

Step 4/5 : CMD ["python3", "/app/app.py"]

 ---> Using cache

Successfully built 9c1a2b3d4e5f

Successfully tagged my-app:optimized

→

-t - Assigns a name and tag to the image for easy reference

Check the size of the newly built optimized image to confirm it is smaller than the original.

Terminal

docker images my-app:optimized

Expected Output

REPOSITORY          TAG           IMAGE ID       CREATED         SIZE

my-app              optimized     9c1a2b3d4e5f   10 seconds ago  45MB

Key Concept

If you remember nothing else, remember: smaller Docker images save time and resources by starting faster and using less storage.

Common Mistakes

Using large base images like 'ubuntu' without checking if a smaller one like 'alpine' can be used

Large base images increase the final image size unnecessarily, slowing down downloads and startups

Choose minimal base images such as 'alpine' or 'scratch' when possible to reduce image size

Copying unnecessary files into the image during build

Extra files increase image size and may expose sensitive data

Use a .dockerignore file to exclude files not needed in the image

Installing packages without cleaning cache or temporary files

This leaves extra data in the image, making it bigger

Use package manager options to avoid cache or remove temporary files in the same RUN step

Summary

Use 'docker images' to check image sizes before and after optimization.

Build images with smaller base images and clean steps to reduce size.

Smaller images start faster and save storage and bandwidth.