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 |
Frequently Asked Questions
Most Viewed Tools
Screen Size Converter — Diagonal Dimension Tool
Calculate screen width and height from diagonal size and aspect ratio. Convert between inches and centimeters for displays, TVs, and monitors with instant dimension calculations.
Use Tool →DPI Calculator — Print Resolution Tool
Calculate DPI (dots per inch), image dimensions, and print sizes. Convert between pixels and physical dimensions for printing and displays.
Use Tool →TOTP Code Generator — 2FA Testing Tool
Generate time-based one-time passwords from a TOTP secret key. Enter your base32 secret, choose a period and digit length, and get the current and next codes with a live countdown timer. Useful for testing and debugging 2FA integrations.
Use Tool →JSONL Formatter — Line-by-Line Validator
Format, validate, and inspect JSON Lines (JSONL) and NDJSON files. Validates each line individually, reports parse errors by line number, outputs compact JSONL or a pretty-print preview, and lets you download the cleaned file.
Use Tool →JSON to Zod — Schema Generator
Generate Zod validation schema code from a JSON sample object. Infers z.string(), z.number(), z.boolean(), z.array(), z.object(), and z.null() types automatically. Handles nested objects, arrays of objects with optional field detection, and outputs copy-ready TypeScript with import and z.infer type alias.
Use Tool →Password Entropy Calculator — Crack Time Estimator
Calculate the information-theoretic bit entropy of any password or API key. Detects character set pools automatically, shows the total number of possible combinations, and estimates crack time across five attack scenarios from rate-limited web logins to GPU cracking clusters.
Use Tool →TLS Cipher Suite Checker — Strength Analyzer
Check TLS protocol version compatibility and cipher suite strength ratings against current best practices. Supports IANA and OpenSSL cipher names — rates each suite as Strong, Weak, or Deprecated and explains why.
Use Tool →Secret Scanner — API Key & Credential Detector
Scan pasted text, code, or config files for accidentally exposed API keys, tokens, passwords, and private keys. Detects 50+ secret types across AWS, GitHub, Stripe, OpenAI, and more — all client-side, nothing leaves your browser.
Use Tool →Related DevOps & Infrastructure Tools
HTTP Header Analyzer — Security & CORS Audit
Parse and analyze HTTP request or response headers. Identifies categories, explains each header, flags missing security headers, and detects duplicates or suspicious values — entirely in your browser.
Use Tool →SSL Certificate Decoder — Expiry & SAN Inspector
Decode X.509 SSL/TLS certificates and RSA private keys in your browser. View subject, issuer, SANs, validity dates, key type, serial number, and SHA-256/SHA-1 fingerprints. Optionally check if a certificate and private key match.
Use Tool →robots.txt Validator — Crawl Rule Checker
Validate your robots.txt file against the Robots Exclusion Protocol. Checks directive syntax, path formats, Crawl-delay values, and Sitemap URLs. Previews crawl rules per user-agent group. Free and runs entirely in your browser.
Use Tool →Port Number Lookup — Common TCP/UDP Service & Protocol Database
Searchable reference for 80+ well-known TCP and UDP ports. Look up any port number or service name to see the official protocol, service description, port range (well-known/registered), and security recommendations for risky ports.
Use Tool →Random User Agent Generator — Browser String Tool
Generate random browser user agent strings for testing and development.
Use Tool →Query String Parser — URL Parameter Decoder
Parse URL query strings into readable key-value pairs. Decode parameters and inspect URL search queries with ease.
Use Tool →Cookie Parser — HTTP Cookie Decoder
Parse HTTP cookie strings into readable key-value pairs. Decode URL-encoded values and inspect cookies from browser requests.
Use Tool →GitHub Actions Validator — Workflow Syntax & CI/CD Security Audit
Validate GitHub Actions workflow YAML for syntax errors, missing required fields, deprecated commands, mutable action refs, outdated action versions, and broken job dependencies. Get per-job results with fix hints in real time.
Use Tool →Share Your Feedback
Help us improve this tool by sharing your experience