Dockerfile Linter — Optimize & Secure Your Container Builds
Lint Dockerfile instructions for best practices, security issues, and layer optimization. Flags unpinned base images, root user, ADD vs COPY, apt-get mistakes, shell-form CMD, and more — with fix guidance for each issue.
How to Use Dockerfile Linter — Optimize & Secure Your Container Builds
How to Use the Dockerfile Linter
Step 1: Paste Your Dockerfile
Copy your entire Dockerfile and paste it into the input area. The linter parses all instructions including multi-line RUN commands joined with backslash continuations.
Step 2: Review the Summary
The summary bar shows the total issue count broken down by severity — errors (critical, must fix), warnings (important best practices), and info (recommendations). The instruction count confirms how many Dockerfile statements were successfully parsed.
Step 3: Expand Issues for Details
Click any issue card to expand it and see a full explanation of why the practice is problematic, plus a concrete fix suggestion you can copy directly into your Dockerfile.
Step 4: Try the Example Dockerfiles
Use the example buttons to see the linter in action on a naive Dockerfile (many issues), a typical Node.js app Dockerfile (some issues), and a production-ready multi-stage Dockerfile (minimal issues). These examples illustrate how each rule applies in practice.
Checks Performed
| Severity | Check | What It Catches |
|---|---|---|
| Error | Missing FROM | Dockerfile with no base image |
| Warning | Unpinned base image | FROM node (no tag) or FROM node:latest |
| Warning | Root user | No USER instruction or explicit USER root |
| Warning | ADD vs COPY | ADD used for local files instead of COPY |
| Warning | apt-get without -y | Interactive prompt would hang the build |
| Warning | apt cache not cleared | rm -rf /var/lib/apt/lists/* missing |
| Warning | apt-get update alone | Standalone update creates stale cache layer |
| Warning | Shell-form CMD/ENTRYPOINT | Poor signal handling, PID 1 is shell |
| Warning | Secrets in ENV | PASSWORD, SECRET, TOKEN, API_KEY in ENV |
| Warning | Source before manifests | Layer cache busted on every code change |
| Info | No WORKDIR | Files land in root directory |
| Info | Many RUN layers | 5+ separate RUN commands bloat image |
| Info | No EXPOSE | Port not documented |
| Info | No HEALTHCHECK | Orchestrators cannot detect unhealthy containers |
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