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dockerfile-basics

@pluginagentmarketplace/custom-plugin-docker
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Learn Dockerfile fundamentals and best practices for building production-ready container images

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SKILL.md

name dockerfile-basics
description Learn Dockerfile fundamentals and best practices for building production-ready container images
sasmp_version 1.3.0
bonded_agent 01-docker-fundamentals
bond_type PRIMARY_BOND

Dockerfile Basics Skill

Master Dockerfile fundamentals and 2024-2025 best practices for building secure, optimized container images.

Purpose

Provide comprehensive guidance on Dockerfile syntax, instruction ordering, layer optimization, and security best practices.

Parameters

Parameter Type Required Default Description
base_image string No - Base image to use
language string No - Programming language (node/python/go/java)
optimize boolean No true Apply optimization recommendations

Core Instructions

Instruction Reference

Instruction Purpose Example
FROM Base image FROM node:20-alpine
WORKDIR Set working directory WORKDIR /app
COPY Copy files COPY package*.json ./
RUN Execute command RUN npm ci
ENV Set environment ENV NODE_ENV=production
EXPOSE Document port EXPOSE 3000
USER Set user USER appuser
CMD Default command CMD ["node", "app.js"]
ENTRYPOINT Fixed command ENTRYPOINT ["./start.sh"]
HEALTHCHECK Health check HEALTHCHECK CMD curl -f http://localhost/

Layer Optimization Order

# 1. Base image (most stable)
FROM node:20-alpine

# 2. System dependencies
RUN apk add --no-cache curl

# 3. Create user (security)
RUN addgroup -g 1001 app && adduser -u 1001 -G app -D app

# 4. Set working directory
WORKDIR /app

# 5. Copy dependency files (cache layer)
COPY package*.json ./

# 6. Install dependencies
RUN npm ci --only=production

# 7. Copy application code (most volatile)
COPY --chown=app:app . .

# 8. Switch to non-root user
USER app

# 9. Health check
HEALTHCHECK --interval=30s --timeout=3s CMD curl -f http://localhost:3000/health || exit 1

# 10. Default command
CMD ["node", "server.js"]

Best Practices (2024-2025)

Security Essentials

# Always use specific version tags
FROM node:20.10-alpine  # Good
# FROM node:latest      # Bad

# Run as non-root user
USER nonroot

# Use multi-stage builds
FROM node:20 AS builder
# ... build steps ...
FROM node:20-alpine AS runtime
COPY --from=builder /app/dist ./

Optimization Techniques

# Combine RUN commands
RUN apt-get update && \
    apt-get install -y --no-install-recommends curl && \
    rm -rf /var/lib/apt/lists/*

# Use .dockerignore
# node_modules, .git, *.md, etc.

# Leverage BuildKit cache mounts
RUN --mount=type=cache,target=/root/.npm npm ci

Error Handling

Common Errors

Error Cause Solution
COPY failed: file not found File outside context Check .dockerignore
returned non-zero code: 127 Command not found Install package first
permission denied Running as non-root Use COPY --chown

Validation Commands

# Lint Dockerfile
hadolint Dockerfile

# Build with no cache
docker build --no-cache -t app:test .

# Inspect layers
docker history app:test

Troubleshooting

Debug Checklist

  • .dockerignore excludes unnecessary files?
  • Base image tag is specific (not :latest)?
  • Dependencies copied before source code?
  • Non-root user configured?
  • HEALTHCHECK defined?

Common Issues

Symptom Cause Fix
Large image size No multi-stage Add build stage
Slow builds Poor layer order Move COPY after dependencies
Security warnings Root user Add USER instruction

Usage

Skill("dockerfile-basics")

Related Skills

  • docker-multi-stage
  • docker-optimization
  • docker-security