How do you optimize Docker image size?
Jul 04, 2025 am 01:23 AM
Using lightweight basic images, merging and optimizing RUN instructions, and copying only necessary files are the key to reducing Docker images size. 1. Select lightweight basic images such as alpine, distroless or scratch to reduce unnecessary system components; 2. Merge multiple RUN commands and clean caches in time, such as combining apt-get update with installation commands, and delete /var/lib/apt/lists/*; 3. Exclude non-essential files through .dockerignore, use multi-stage construction to separate compilation and runtime dependencies, and copy only the necessary configuration and executable files into the final image. These methods can effectively reduce mirror size, improve construction and deployment efficiency, and reduce security risks.
Optimizing Docker image size is mostly about being intentional with what goes into the image and how you build it. A smaller image means faster builds, quicker deployments, and a reduced attack surface — all worth aiming for.
Use a Lightweight Base Image
One of the easiest ways to cut down on image bloat is to start with a small base image.
For example, using alpine variants (like python:3.11-alpine ) can hold hundreds of megabytes off your final image. They're stripped-down versions of full OS images but still functional enough for most services.
If Alpine doesn't work for your stack (some packages are harder to compile on it), consider alternatives like distroless or scratch for truly minimal setups — especially useful in production-ready binaries like Go apps.
Just keep in mind:
- Smaller images might mean fewer debugging tools
- Some libraries expect certain system components that minimal images don't include
So pick one that fits your runtime needs without dragging in unnecessary extras.
Combine and Optimize RUN Instructions
Each RUN , COPY , or ADD instruction creates a new layer in the image. Too many layers mean more overhead and larger images.
The trick here is to group related commands together.
Instead of doing this:
RUN apt-get update RUN apt-get install -y curl
Do this:
RUN apt-get update && \
apt-get install -y curl && \
rm -rf /var/lib/apt/lists/*This reduces layers and cleans up cache files right away so they don't stick around in the final image.
Also, chain commands smartly:
- Always clean up package caches (
apt-get clean,yum clean all, etc.) - Delete temporary files or directories created during setup
- Avoid installing dev dependencies unless needed at runtime
Only COPY What You Need
It's common to see COPY . . used liberally, but copying everything increases the image size unnecessarily.
Be selective.
Use .dockerignore to exclude test files, logs, node_modules (if building inside Docker), and other non-essential assets.
For example, in a Node.js app:
COPY package*.json ./ RUN npm install COPY --from=builder /app/dist ./dist
This limits what gets copied and separates build-time from runtime dependencies, especially when using multi-stage builds.
And if you're working with compiled languages ??like Go or Java, build the binary first and only copy the executable config files — no source code needed in the final image.
That's basically it. The main idea is to stay aware of what's going into each layer and avoid carrying extra baggage. It's not complicated, but it makes a real difference when done consistently.
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