Docker Lint Skill

📋 Overview

This skill uses hadolint (Dockerfile linter) to check Docker image build files for best practices, ensuring:

•🔒 Security (non-root user, minimal privileges)
•⚡ Performance optimization (layer caching, multi-stage builds)
•📏 Standard compliance (Docker official best practices)
•🐛 Common error detection (typos, invalid instructions)

🔧 Prerequisites

Tool	Min Version	Check Command	Installation
Docker	20.10+	`docker --version`	docker.com
hadolint	2.12+	`hadolint --version`	See installation below

Installing hadolint

Windows (Scoop recommended):

powershell

scoop install hadolint

Linux:

bash

wget -O /usr/local/bin/hadolint https://github.com/hadolint/hadolint/releases/latest/download/hadolint-Linux-x86_64
chmod +x /usr/local/bin/hadolint

macOS:

bash

brew install hadolint

Docker (all platforms):

bash

docker pull hadolint/hadolint

Note: The script will auto-detect and prompt for installation, supporting Docker container run mode.

🚀 Usage

Method 1: Use AI Assistant

code

"Use docker-lint skill to check my Dockerfile"

Method 2: Run Script Directly

Check single Dockerfile:

powershell

# Windows
.\.agent\skills\docker-lint\scripts\lint.ps1

# Linux/Mac
./.agent/skills/docker-lint/scripts/lint.sh

Check specific file:

powershell

# Windows
.\.agent\skills\docker-lint\scripts\lint.ps1 -File ".\docker\Dockerfile.prod"

# Linux/Mac
./.agent/skills/docker-lint/scripts/lint.sh docker/Dockerfile.prod

Check all Dockerfiles in directory:

powershell

# Windows
.\.agent\skills\docker-lint\scripts\lint.ps1 -Path ".\containers" -Recursive

# Linux/Mac
./.agent/skills/docker-lint/scripts/lint.sh -r containers/

🎯 What It Checks

Security Checks

•✅ DL3002: Prohibit running container as root user
•✅ DL3008: Pin apt-get package versions
•✅ DL3013: Pin pip package versions
•✅ DL3059: Multi-stage build health check
•✅ SC2046: Shell script injection protection

Performance Optimization

•✅ DL3003: Use WORKDIR instead of cd
•✅ DL3009: Clean apt cache
•✅ DL3015: Avoid unnecessary package updates
•✅ DL3020: Use COPY instead of ADD
•✅ DL3045: Layer cache optimization

Standard Compliance

•✅ DL3006: Specify base image tag
•✅ DL3007: Avoid using latest tag
•✅ DL3025: Use JSON format for CMD/ENTRYPOINT
•✅ DL4000: MAINTAINER is deprecated

📊 Output Example

code

🐳 Docker Lint - Checking Dockerfile...

━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
📁 File: Dockerfile
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

Dockerfile:1 DL3006 warning: Always tag the version of an image explicitly
FROM python:3
     ^


Dockerfile:5 DL3008 warning: Pin versions in apt-get install
RUN apt-get update && apt-get install -y git
                                         ^


Dockerfile:15 DL3002 error: Last USER should not be root
USER root
^

━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
📊 Check Results:
   ❌ Errors: 1
   ⚠️  Warnings: 2
   💡 Info: 0
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

💡 Suggestions:
1. Pin base image version: FROM python:3.11-slim
2. Pin apt package version: git=1:2.34.1-1ubuntu1.10
3. Use non-root user: USER appuser

⚙️ Configuration

Create .hadolint.yaml in the project root to customize rules:

yaml

# .hadolint.yaml
ignored:
  - DL3008  # Allow unpinned apt package versions (dev environment)

trustedRegistries:
  - docker.io
  - gcr.io
  - ghcr.io

label-schema:
  author: email
  version: semver

# Custom severity
override:
  error:
    - DL3002  # root user is error level
  warning:
    - DL3008  # unpinned version is warning level
  info:
    - DL3015  # package update suggestion is info level

🛠️ Dockerfile Fix Example

Problem Dockerfile:

dockerfile

FROM python:3
RUN apt-get update && apt-get install -y git
COPY . /app
WORKDIR /app
RUN pip install -r requirements.txt
USER root
CMD python app.py

Fixed Dockerfile:

dockerfile

# Pin base image version
FROM python:3.11-slim

# Create non-root user
RUN groupadd -r appuser && useradd -r -g appuser appuser

# Pin package versions and clean cache
RUN apt-get update && \
    apt-get install -y --no-install-recommends \
        git=1:2.34.1-1ubuntu1.10 && \
    rm -rf /var/lib/apt/lists/*

# Set working directory
WORKDIR /app

# Copy dependency file first (leverage caching)
COPY requirements.txt .
RUN pip install --no-cache-dir -r requirements.txt

# Then copy application code
COPY --chown=appuser:appuser . .

# Switch to non-root user
USER appuser

# Use JSON format
CMD ["python", "app.py"]

🔗 CI/CD Integration

GitHub Actions

yaml

name: Lint Dockerfile
on: [push, pull_request]

jobs:
  hadolint:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: hadolint/hadolint-action@v3.1.0
        with:
          dockerfile: Dockerfile

GitLab CI

yaml

hadolint:
  image: hadolint/hadolint:latest-alpine
  script:
    - hadolint Dockerfile

🆘 FAQ

Q: What if hadolint is not installed?
A: The script will automatically try to run hadolint using Docker container

Q: How to ignore specific rules?
A: Add a comment in the Dockerfile:

dockerfile

# hadolint ignore=DL3008
RUN apt-get install -y git

Q: Does it support multi-stage builds?
A: Fully supported, hadolint checks best practices for each stage

Q: Can it check docker-compose.yml?
A: hadolint focuses on Dockerfile, use docker-compose config --quiet for docker-compose validation