Reducing final image size by 80 percent in Docker - Commands & Configuration

Introduction

Large Docker images take longer to download and use more storage. Reducing the image size by 80 percent helps apps start faster and saves space on servers.

When you want your app to start quickly on any machine.

When you have limited storage on your server or cloud.

When you want to reduce data transfer time during deployments.

When you want to keep your Docker images clean and efficient.

When you want to avoid including unnecessary files or tools in your app image.

Config File - Dockerfile

Dockerfile

FROM python:3.11-slim
WORKDIR /app
COPY requirements.txt .
RUN pip install --no-cache-dir -r requirements.txt
COPY . .
CMD ["python", "app.py"]

This Dockerfile uses a slim Python base image to keep the size small.

It installs only required packages without caching to avoid extra files.

It copies only necessary files into the image.

The CMD runs the Python app.

Commands

Builds the Docker image using the Dockerfile that reduces size by using a slim base and no cache during install.

Terminal

docker build -t my-app:small .

Expected Output

Sending build context to Docker daemon  5.12MB

Step 1/5 : FROM python:3.11-slim

 ---> 123abc456def

Step 2/5 : WORKDIR /app

 ---> Using cache

 ---> 789def012abc

Step 3/5 : COPY requirements.txt .

 ---> Using cache

 ---> 345abc678def

Step 4/5 : RUN pip install --no-cache-dir -r requirements.txt

 ---> Running in 456def789abc

Collecting flask

Installing collected packages: flask

Successfully installed flask-2.3.2

Removing intermediate container 456def789abc

 ---> 901abc234def

Step 5/5 : COPY . .

 ---> 567def890abc

Successfully built 567def890abc

Successfully tagged my-app:small

→

-t - Names the image for easy reference

Shows the size of the newly built small image to verify the reduction.

Terminal

docker images my-app:small

Expected Output

REPOSITORY   TAG       IMAGE ID       CREATED          SIZE

my-app       small     567def890abc   10 seconds ago   45MB

Runs the small image to confirm the app works correctly after size reduction.

Terminal

docker run --rm my-app:small

Expected Output

 * Running on http://0.0.0.0:5000/ (Press CTRL+C to quit)

→

--rm - Automatically removes the container after it stops

Key Concept

If you remember nothing else from this pattern, remember: use slim base images and avoid caching to drastically reduce Docker image size.

Common Mistakes

Using a full base image like python:3.11 instead of python:3.11-slim

Full base images include unnecessary tools and files, making the image much larger.

Use slim or alpine base images that contain only essential components.

Not using --no-cache-dir when installing packages

Pip caches installation files inside the image, increasing size unnecessarily.

Add --no-cache-dir flag to pip install commands to avoid caching.

Copying the entire project including unnecessary files like docs or tests

Unneeded files increase image size and slow down builds.

Use .dockerignore to exclude unnecessary files or copy only needed files explicitly.

Summary

Use a slim base image to start with a smaller foundation.

Install packages without cache to avoid extra files in the image.

Copy only necessary files to keep the image clean and small.

Verify image size with docker images command.

Run the image to ensure the app still works after size reduction.