Docker Helper

Overview

Create, debug, and optimize Docker configurations including Dockerfiles, docker-compose setups, and container troubleshooting. Follows Docker best practices for security, image size, and build speed.

Instructions

When a user asks for help with Docker, determine which task they need:

Task A: Create a Dockerfile

1. Identify the application language and framework
2. Choose the right base image (prefer -slim or -alpine variants)
3. Use multi-stage builds for compiled languages
4. Follow this ordering for optimal layer caching:

# Stage 1: Build
FROM node:20-alpine AS builder
WORKDIR /app
COPY package*.json ./
RUN npm ci --only=production
COPY . .
RUN npm run build

# Stage 2: Runtime
FROM node:20-alpine
WORKDIR /app
RUN addgroup -g 1001 appgroup && adduser -u 1001 -G appgroup -D appuser
COPY --from=builder /app/dist ./dist
COPY --from=builder /app/node_modules ./node_modules
COPY --from=builder /app/package.json ./
USER appuser
EXPOSE 3000
CMD ["node", "dist/index.js"]

Key rules:

• Copy dependency files first, then source (caching)
• Run as non-root user
• Use .dockerignore to exclude node_modules, .git, .env
• Pin base image versions (e.g., node:20.11-alpine, not node:latest)
• Combine RUN commands to reduce layers

Task B: Write docker-compose.yml

version: "3.8"

services:
  app:
    build:
      context: .
      dockerfile: Dockerfile
    ports:
      - "3000:3000"
    environment:
      - DATABASE_URL=postgres://user:pass@db:5432/myapp
    depends_on:
      db:
        condition: service_healthy
    restart: unless-stopped

  db:
    image: postgres:16-alpine
    volumes:
      - pgdata:/var/lib/postgresql/data
    environment:
      POSTGRES_USER: user
      POSTGRES_PASSWORD: pass
      POSTGRES_DB: myapp
    healthcheck:
      test: ["CMD-SHELL", "pg_isready -U user -d myapp"]
      interval: 5s
      timeout: 5s
      retries: 5

volumes:
  pgdata:

Key rules:

• Use depends_on with health checks, not just service names
• Use named volumes for persistent data
• Never hardcode secrets in compose files for production (use env files or secrets)
• Set restart policies

Task C: Debug container issues

Follow this diagnostic sequence:

# 1. Check container status
docker ps -a

# 2. Read container logs
docker logs <container> --tail 50

# 3. Inspect container details
docker inspect <container> | jq '.[0].State'

# 4. Check resource usage
docker stats --no-stream

# 5. Get a shell inside the container
docker exec -it <container> /bin/sh

# 6. Check networking
docker network ls
docker network inspect <network>

Common issues and fixes:

• Container exits immediately: Check logs; usually a missing env var or config file
• Port already in use: lsof -i :PORT to find the conflict
• Cannot connect between containers: Ensure they are on the same Docker network and use service names as hostnames
• Out of disk space: docker system prune -a to clean up unused images and containers
• Build fails at COPY: Check .dockerignore and verify paths are relative to build context

Task D: Optimize image size

# Check current image size
docker images | grep myapp

# Analyze layers
docker history myapp:latest

Optimization checklist:

1. Switch to alpine base images
2. Use multi-stage builds (build deps stay in builder stage)
3. Combine RUN commands with &&
4. Remove package manager caches: rm -rf /var/cache/apk/*
5. Use .dockerignore to exclude test files, docs, .git
6. Do not install dev dependencies in the final image

Examples

Example 1: Create a Dockerfile for a Python FastAPI app

User request: "Create a Dockerfile for my FastAPI project"

Output Dockerfile:

FROM python:3.12-slim AS builder
WORKDIR /app
COPY requirements.txt .
RUN pip install --no-cache-dir --prefix=/install -r requirements.txt

FROM python:3.12-slim
WORKDIR /app
RUN useradd -r -s /bin/false appuser
COPY --from=builder /install /usr/local
COPY . .
USER appuser
EXPOSE 8000
CMD ["uvicorn", "main:app", "--host", "0.0.0.0", "--port", "8000"]

Output .dockerignore:

__pycache__
*.pyc
.git
.env
.venv
tests/
*.md

Result: Image size ~180MB instead of ~1.2GB with the full Python image.

Example 2: Debug a container that keeps restarting

User request: "My web container keeps restarting, can you help?"

Diagnostic steps:

$ docker ps -a
CONTAINER ID  IMAGE    STATUS                     NAMES
a1b2c3d4e5f6  web:1.0  Restarting (1) 5 sec ago   web

$ docker logs web --tail 20
Error: ECONNREFUSED 127.0.0.1:5432
    at TCPConnectWrap.afterConnect

Root cause: The app is trying to connect to PostgreSQL at 127.0.0.1,
but in Docker the database is a separate container.

Fix: Change DATABASE_URL from localhost to the service name:
  DATABASE_URL=postgres://user:pass@db:5432/myapp
                                     ^^ service name, not localhost

Guidelines

• Always create a .dockerignore alongside any Dockerfile.
• Pin base image versions to specific tags, not latest.
• Never store secrets in Docker images or Dockerfiles. Use environment variables or Docker secrets.
• For production, always run as a non-root user.
• When optimizing, measure before and after with docker images.
• If a build is slow, check layer ordering. Dependencies (package.json, requirements.txt) should be copied and installed before source code.
• For local development, use bind mounts for live reloading. For production, use COPY.
• Health checks should test actual application readiness, not just process existence.