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

Identify the application language and framework
Choose the right base image (prefer -slim or -alpine variants)
Use multi-stage builds for compiled languages
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:

Switch to alpine base images
Use multi-stage builds (build deps stay in builder stage)
Combine RUN commands with &&
Remove package manager caches: rm -rf /var/cache/apk/*
Use .dockerignore to exclude test files, docs, .git
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.

docker-helper