Reducing image size strategies in Docker - Commands & Configuration

Introduction

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

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

When you have limited storage space on your server or local machine.

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 base image to keep size small.

It installs Python packages without cache to avoid leftover files.

It copies only necessary files and sets the command to run the app.

Commands

Builds the Docker image using the Dockerfile in the current folder and tags it as 'my-app:small'.

Terminal

docker build -t my-app:small .

Expected Output

Sending build context to Docker daemon  12.3MB

Step 1/6 : FROM python:3.11-slim

 ---> 123abc456def

Step 2/6 : WORKDIR /app

 ---> Using cache

 ---> 789def012abc

Step 3/6 : COPY requirements.txt .

 ---> Using cache

 ---> 456abc789def

Step 4/6 : RUN pip install --no-cache-dir -r requirements.txt

 ---> Running in 1a2b3c4d5e6f

Collecting flask

  Downloading Flask-2.3.2-py3-none-any.whl (96 kB)

Installing collected packages: flask

Successfully installed flask-2.3.2

Removing intermediate container 1a2b3c4d5e6f

 ---> 0f1e2d3c4b5a

Step 5/6 : COPY . .

 ---> 9a8b7c6d5e4f

Step 6/6 : CMD ["python", "app.py"]

 ---> Running in 7f8e9d0c1b2a

Removing intermediate container 7f8e9d0c1b2a

 ---> 3c2b1a0f9e8d

Successfully built 3c2b1a0f9e8d

Successfully tagged my-app:small

Shows the size of the newly built image to verify it is small.

Terminal

docker images my-app:small

Expected Output

REPOSITORY   TAG       IMAGE ID       CREATED          SIZE

my-app       small     3c2b1a0f9e8d   10 seconds ago   45MB

Runs the small image to check the app works correctly.

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 to save space.

Key Concept

If you remember nothing else from this pattern, remember: use slim base images and clean up caches to keep Docker images small and efficient.

Common Mistakes

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

Full images include many extra tools and files, making the image much larger than needed.

Choose slim or minimal base images that have only what your app needs.

Not using '--no-cache-dir' when installing Python packages.

Pip caches packages by default, which increases image size unnecessarily.

Add '--no-cache-dir' to pip install commands to avoid saving cache files.

Copying the entire project including unnecessary files like tests or docs.

Unneeded files increase image size and slow down builds and deployments.

Use a .dockerignore file or carefully copy only required files.

Summary

Use slim base images to reduce unnecessary files in your Docker image.

Install packages without cache to avoid leftover files increasing image size.

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

Verify image size with 'docker images' and test the image by running it.