10 Proven Techniques to Shrink Docker Image Size for Faster CI/CD

What is Docker?

Docker is a container engine that runs applications in isolated environments. Images are built from a Dockerfile containing instructions such as FROM, RUN, COPY, EXPOSE, etc.

Why Reduce Docker Image Size?

Unnecessary packages increase the attack surface.

Larger images take longer to transfer.

Deploying large images consumes more time.

Techniques to Reduce Docker Image Size

1. Minimize Image Layers

Combine multiple commands into a single RUN to reduce the number of layers.

FROM ubuntu:latest
RUN apt update -y && \
    apt install -y unzip curl python3 --no-install-recommends && \
    rm -rf /var/lib/apt/lists/*

2. Use Docker Squash

Install docker-squash and squash layers into a single layer.

pip install docker-squash
docker-squash image:old -t image:new

3. Choose Smaller Base Images

Prefer slim or Alpine variants. Example sizes: python:3.9 ≈ 1.3 GB, python:3.9-slim ≈ 1 GB, python:3.9-alpine ≈ 49 MB.

4. Multi‑Stage Builds

Separate build and runtime stages so that only the final artifacts are copied into the runtime image.

# Stage 1 – build
FROM node:14.17-alpine3.14 AS build
COPY public /home/app/public/
COPY src /home/app/src/
RUN apk add --no-cache g++ make python2 \
    && npm install --silent \
    && npm run build \
    && apk del python2

# Stage 2 – runtime
FROM nginx:stable-alpine
COPY nginx.conf /etc/nginx/conf.d/default.conf
COPY --from=build /home/app/build /usr/share/nginx/html
EXPOSE 80
CMD ["nginx", "-g", "daemon off;"]

5. Install Packages Without Recommended Extras

Use --no-install-recommends (apt) or --no-cache (apk) to avoid unnecessary packages.

FROM ubuntu:latest
RUN apt update -y && \
    apt install -y unzip curl python3 --no-install-recommends && \
    rm -rf /var/lib/apt/lists/*

6. Clean Package Manager Cache

Delete the apt cache after installation to free space.

RUN rm -rf /var/lib/apt/lists/*

7. Use a .dockerignore File

Exclude files that are not needed in the build context.

ignorethisfile.txt
logs/
ignorethisfolder/
.git
.cache
*.md

8. Order COPY After Heavy RUN Steps

Placing COPY after RUN allows Docker to cache the expensive steps and rebuild faster.

# Better
FROM ubuntu:latest
RUN apt update -y && \
    apt install -y unzip curl python3 --no-install-recommends && \
    rm -rf /var/lib/apt/lists/*
COPY file /home/ubuntu

# Worse
FROM ubuntu:latest
COPY file /home/ubuntu
RUN apt update -y && \
    apt install -y unzip curl python3 --no-install-recommends && \
    rm -rf /var/lib/apt/lists/*

9. Delete Installation Artifacts

Remove downloaded archives after they are used.

FROM ubuntu:latest
RUN curl -L "https://awscli.amazonaws.com/awscli-exe-linux-x86_64.zip" -o awscliv2.zip && \
    unzip awscliv2.zip && \
    ./aws/install && \
    rm awscliv2.zip

10. Analyse Image Size with Tools

Tools such as Dive (https://github.com/wagoodman/dive), fromlatest.io (https://www.fromlatest.io/), and Docker Slim (https://github.com/slimtoolkit/slim) can inspect layers and suggest further reductions.