When using Docker, one of the main goals is to create container images that are as compact, secure, and high-performance as possible. This is especially important in production environments, CI/CD pipelines, cloud, and serverless setups where every megabyte matters.
One tool that helps achieve this is docker-slim.
In this article, we’ll explain what docker-slim is, how it works, and why you should add it to your DevOps toolbox.
Why Minimize Docker Images?
The standard Dockerfile-based approach often results in bloated images containing:
- Unused runtime dependencies (compilers, test utilities)
- Debugging tools
- Temporary files
- Packages installed “just in case”
This leads to:
- Slower build and delivery times
- Larger attack surface (more tools = more vulnerabilities)
- Increased storage and network costs
What Does docker-slim Do?
docker-slim is a CLI tool that analyzes how your container behaves during runtime and builds a minimally required Docker image by removing everything unnecessary.
It works by:
- Launching your container in “inspection” mode
- Monitoring which files and libraries the application actually uses
- Creating a new, optimized image that includes only those files
Benefits
- Security — Fewer potential vulnerabilities
- Size — Images can shrink by 10–30x
- Simplicity — No need to manually fine-tune base images
- Compatibility — Works with most languages (Python, Node.js, Go, Java, etc.)
How to Use docker-slim
Example for a Python app:
docker build -t myapp .
docker-slim build myappResult:
- Original image: myapp
- Optimized image: myapp.slim
Example output:
docker images
REPOSITORY TAG IMAGE ID SIZE
myapp latest abc123 350MB
myapp.slim latest def456 18MB
Advanced Features
- http-probe — Automatically generates HTTP requests to trigger all app routes
- user-probes — Define custom probes (scripts, curl, etc.)
- CI/CD integration — Easily use with GitHub Actions, GitLab CI, and more
- Security analysis — Highlights risks and dependency issues
Limitations
- The app must actually run in inspection mode
- Dynamic dependencies may be missed if not triggered during analysis (can be resolved with custom probes)
- Not ideal for apps with heavy initialization or GUI/headless UIs
Conclusion
docker-slim is a powerful and convenient tool for automatically optimizing Docker images. It significantly reduces image size, boosts security, and speeds up deployments — all without requiring manual Dockerfile rewrites. It’s especially valuable in production, microservices, cloud, and CI/CD environments.
If you’re not using docker-slim yet, now is a great time to try it. It’s a small step that can make a huge difference in the quality and efficiency of your infrastructure.
