How to Fix Hadolint Dockerfile Lint Error

In this tutorial, you'll learn about How to Fix Hadolint Dockerfile Lint Error. We cover key concepts, practical examples, and best practices.

Hadolint returns multiple lint errors when analyzing a Dockerfile, such as DL3008: Pin versions in apt-get install or DL3018: Pin package versions in apk add — the Dockerfile violates best practice rules for building container images.

The Problem

$ hadolint Dockerfile
Dockerfile:5 DL3008 error: Pin versions in apt-get install
Dockerfile:7 DL3018 error: Pin versions in apk add
Dockerfile:12 DL4006 warning: Use COPY instead of ADD

Step-by-Step Fix

Step 1: Pin package versions

# Bad
RUN apt-get update && apt-get install -y curl

# Good
RUN apt-get update && apt-get install -y curl=7.88.1-10+deb12u1

Step 2: Use COPY instead of ADD

# Bad
ADD ./app /app

# Good
COPY ./app /app

Step 3: Avoid sudo in RUN commands

# Bad
RUN sudo apt-get install -y curl

# Good
RUN apt-get install -y curl

Step 4: Remove apt-get cache

# Bad
RUN apt-get update && apt-get install -y curl

# Good
RUN apt-get update && apt-get install -y curl && rm -rf /var/lib/apt/lists/*

Step 5: Use WORKDIR instead of cd

# Bad
RUN cd /app && npm install

# Good
WORKDIR /app
RUN npm install

Step 6: Configure hadolint

# .hadolint.yaml
failure-threshold: warning

override:
  - image: node:20-alpine
    rules:
      - DL3008 = false  # Skip version pinning for base image

Step 7: Run with ignore

hadolint --ignore DL3008 Dockerfile

Prevention Tips

• Install hadolint as a pre-commit hook
• Pin all package versions in package managers
• Always use COPY instead of ADD for local files
• Run hadolint Dockerfile in CI/CD pipelines

Common Mistakes with dockerfile

1. Non-exhaustive pattern matches that compile with warnings then crash at runtime
2. Misunderstanding that String is [Char] with poor performance for large text operations
3. Using foldl instead of foldl' causing stack overflow on large lists

These mistakes appear frequently in real-world HADOLINT code. DodaTech's contributors have identified these patterns through analysis of open-source projects and production systems.

Practice Exercise

Write a pure function that safely divides two integers using Maybe, then test it with edge cases like division by zero and negative numbers.

This exercise reinforces the concepts covered in this guide. Try implementing it before checking online solutions.

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