Image size and minimal base images in Docker - Commands & Configuration

Introduction

Large Docker images take longer to download and use more storage. Using minimal base images helps keep images small and efficient, making your apps start faster and use fewer resources.

When you want your app to start quickly in a cloud environment with limited bandwidth.

When you need to save disk space on servers running many containers.

When you want to reduce security risks by including fewer unnecessary packages.

When you want faster deployment times during continuous integration and delivery.

When you want to optimize resource usage on development machines.

Config File - Dockerfile

Dockerfile

FROM alpine:3.18

RUN apk add --no-cache python3 py3-pip

COPY app.py /app/app.py

CMD ["python3", "/app/app.py"]

FROM alpine:3.18 sets a very small base image with just essential tools.

RUN apk add --no-cache python3 py3-pip installs Python without caching to keep the image small.

COPY app.py /app/app.py copies your application code into the image.

CMD runs the Python app when the container starts.

Commands

Builds the Docker image named 'my-python-app' from the Dockerfile in the current directory.

Terminal

docker build -t my-python-app .

Expected Output

Sending build context to Docker daemon  3.072kB

Step 1/4 : FROM alpine:3.18

3.18: Pulling from library/alpine

Digest: sha256:...

Status: Downloaded newer image for alpine:3.18

 ---> 3f53bb00af94

Step 2/4 : RUN apk add --no-cache python3 py3-pip

 ---> Running in abcdef123456

fetch http://dl-cdn.alpinelinux.org/alpine/v3.18/main/x86_64/APKINDEX.tar.gz

(1/7) Installing python3 (3.11.4-r0)

(2/7) Installing py3-pip (23.1.2-r0)

Executing busybox-1.35.0-r21.trigger

OK: 15 MiB in 29 packages

Removing intermediate container abcdef123456

 ---> 123456abcdef

Step 3/4 : COPY app.py /app/app.py

 ---> 789abc123def

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

 ---> Running in 456def789abc

Removing intermediate container 456def789abc

 ---> fedcba987654

Successfully built fedcba987654

Successfully tagged my-python-app:latest

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

Terminal

docker images my-python-app

Expected Output

REPOSITORY      TAG       IMAGE ID       CREATED          SIZE

my-python-app   latest    fedcba987654   10 seconds ago   30MB

Runs the container from the image to check the app works as expected.

Terminal

docker run --rm my-python-app

Expected Output

Hello from app.py

→

--rm - Automatically removes the container after it stops to keep the system clean

Key Concept

If you remember nothing else from this pattern, remember: using minimal base images like Alpine keeps your Docker images small and efficient.

Common Mistakes

Using large base images like 'ubuntu' without considering size.

This makes images bulky, slow to download, and wastes storage.

Choose minimal base images like 'alpine' or 'scratch' when possible.

Installing packages without cleaning cache or using no-cache flags.

This leaves unnecessary files in the image, increasing size.

Use package manager flags like '--no-cache' in Alpine to avoid cache files.

Copying unnecessary files into the image.

Extra files increase image size and may expose sensitive data.

Only copy needed files and use .dockerignore to exclude others.

Summary

Use minimal base images like Alpine to reduce Docker image size.

Install only necessary packages with no-cache options to avoid leftover files.

Verify image size with 'docker images' and test the container runs correctly.