🐳 Dockerfile Linter

What Can the Dockerfile Linter Do?

The Dockerfile Linter is a free online tool that checks your Dockerfile against common best practices. It is built on the Docker specification and the Hadolint rule set, and detects security vulnerabilities, image size optimization opportunities, and other common issues — helping you build smaller, safer, and more reliable container images.

Core Features

• 30+ Check Rules: security, image optimization, and maintainability dimensions
• Three Severity Levels: error (must fix), warning (recommended), info (optimization tip)
• Real-Time Checking: checks as you type — no button click needed
• Concrete Fix Suggestions: each issue includes a code example you can copy
• Quality Score: a 0–100 score that summarizes your Dockerfile quality
• Drag & Drop: check a Dockerfile file directly

How to Use

1. Paste Dockerfile content into the input, or drag a Dockerfile file to upload it

2. Click the "Check" button or press Ctrl+Enter to start checking

3. View the check results: quality score, issue count, and a detailed issue list

4. Click each issue to view its detailed description and fix suggestion

5. Click "Copy suggestion" to copy all suggestions at once

6. Use "Load Example" to see a sample Dockerfile and its issues

Use Cases

Use Case 1: DevOps Review

In a CI/CD pipeline, DevOps engineers use the linter to review Dockerfiles before merge, ensuring they meet security and best-practice standards so insecure images never ship.

Use Case 2: Image Size Optimization

For bloated Docker images, the linter surfaces unmerged RUN layers, uncleaned apt caches, and non-alpine base images. Fixing these issues can reduce image size by over 60%.

Use Case 3: Security Audit

Run a container security audit using the linter to quickly spot running as root, exposed port 22, and using :latest tags — then generate a complete fix list.

Use Case 4: Learning Docker

New Docker users load the example to learn common Dockerfile mistakes and best practices, quickly learning to write high-quality Dockerfiles.

Best Practices

A Dockerfile is the blueprint of a Docker image — its quality directly impacts security, image size, and startup speed. Key best practices include: using multi-stage builds to shrink the final image; choosing lightweight base images like Alpine; merging RUN instructions to reduce layers; using COPY instead of ADD to avoid accidental extraction; running as a non-root user; adding a .dockerignore to exclude unnecessary files; and pinning base image versions instead of using :latest. These practices improve security and deployment reliability.