Docker

Docker Best Practices (2025)

First Question: Do You Need a Dockerfile?

For fast-moving projects, the fastest Docker setup has no Dockerfile at all.

Do you need a Dockerfile?
│
├─ Deploying to your own swarm/server?
│  └─ NO - Use mounted code pattern (see below)
│
├─ Publishing to Docker Hub / registry?
│  └─ YES - Others need to build your image
│
├─ CI/CD building images?
│  └─ YES - Need reproducible builds
│
├─ Complex native dependencies?
│  └─ MAYBE - If mount + base image isn't enough
│
├─ Open source project?
│  └─ YES - Contributors need to build/run
│
└─ Just deploying your own code?
   └─ NO - Mount it, move fast

Fast Path: Mounted Code Pattern

Build locally, mount into runtime container. No Dockerfile, no registry, instant deploys.

# docker-compose.yaml - the "no Dockerfile" approach
services:
  app:
    image: node:22-slim
    working_dir: /app
    volumes:
      - /absolute/path/to/project:/app:ro
    command: node dist/index.js
    environment:
      - NODE_ENV=production

Why this works:

• pnpm build locally → mount dist/ into container
• Code changes = rebuild + restart (no image rebuild)
• Base images updated by just pulling latest
• Perfect for internal services, hobby projects, fast iteration

When to graduate to Dockerfiles:

• Image size matters (multi-stage shrinks significantly)
• Need CI/CD to build images
• Publishing for others to use
• Native dependencies that need build-time compilation

When You DO Need a Dockerfile

Required Reading by Topic

⚠️ BEFORE providing Dockerfile guidance, read the relevant reference:

Skipping references leads to outdated patterns. Docker best practices change frequently.

5-Minute Essentials (Always Do These)

These take 5 minutes and prevent 80% of problems:

1. Pin your base image (30 sec)

# BAD
FROM node:latest

# GOOD
FROM node:22-slim

2. Add .dockerignore (2 min)

.git
node_modules
dist
.env*
*.log

Copy template: assets/dockerignore.template

3. Non-root user (2 min)

# Add before final CMD
RUN groupadd -r app && useradd -r -g app app
USER app

4. Order layers correctly (30 sec)

# Dependencies first (cached)
COPY package*.json ./
RUN npm ci

# Code last (changes often)
COPY . .

That's it for fast-moving projects. Everything below is for when you need to optimize further.

Comprehensive Optimization (When Publishing/Scaling)

Multi-Stage Builds

Separate build-time from runtime for smaller images:

# syntax=docker/dockerfile:1

FROM node:22 AS builder
WORKDIR /app
COPY package*.json ./
RUN npm ci
COPY . .
RUN npm run build

FROM node:22-slim AS production
WORKDIR /app
RUN groupadd -r app && useradd -r -g app app
COPY --from=builder --chown=app:app /app/dist ./dist
COPY --from=builder --chown=app:app /app/node_modules ./node_modules
USER app
CMD ["node", "dist/index.js"]

→ See: references/multi-stage-patterns.md for advanced patterns

BuildKit Cache Mounts

Speed up dependency installation:

# syntax=docker/dockerfile:1
RUN --mount=type=cache,target=/root/.npm npm ci

→ See: references/buildkit-features.md for cache mounts, secrets, COPY --link

Base Image Selection

Apple Silicon Limitation: Multi-stage Go builds with docker buildx --platform linux/amd64 segfault on Apple Silicon Macs due to QEMU issues with Go 1.24+. Use native cross-compilation instead:

GOOS=linux GOARCH=amd64 CGO_ENABLED=0 go build -o app .
# Then use simple Dockerfile that just copies the binary

See assets/go-prebuilt.dockerfile for the pattern.

→ See: references/base-images.md for detailed selection guide

Language-Specific Templates

→ See: references/language-patterns.md for detailed patterns

Dockerfile Audit Checklist

When reviewing any Dockerfile:

Must Fix (Security)

• Non-root USER instruction
• No secrets in ENV or COPY
• Pinned base image versions (no :latest)
• .dockerignore excludes .env, .git, secrets

Should Fix (Size/Speed)

• Multi-stage separates build from runtime
• Layer order: deps first, code last
• Uses *-slim or smaller base

Nice to Have

• BuildKit cache mounts
• COPY --link for multi-stage
• HEALTHCHECK defined
• OCI labels for metadata

Quick Reference

Common Anti-Patterns

# BAD: Everything wrong
FROM node:latest
COPY . .
RUN npm install
ENV DATABASE_URL=postgres://secret
CMD ["npm", "start"]

# GOOD: Fixed
FROM node:22-slim
WORKDIR /app
COPY package*.json ./
RUN npm ci --only=production
COPY . .
RUN groupadd -r app && useradd -r -g app app
USER app
CMD ["node", "dist/index.js"]

Size Targets

Build Time Targets

Integration with Swarm

If using Docker Swarm with Traefik:

Mounted code (no Dockerfile):

services:
  app:
    image: node:22-slim
    volumes:
      - /absolute/path:/app:ro
    deploy:
      labels:
        - "traefik.enable=true"
        - "traefik.http.routers.myapp.rule=Host(`myapp.example.com`)"

Built image (with Dockerfile):

services:
  app:
    image: myregistry/myapp:1.0.0
    deploy:
      labels:
        - "traefik.enable=true"
        # ...

Decision Trees

Creating New Project

Need Docker?
├─ Internal service, fast iteration → Mounted code, no Dockerfile
├─ Publishing to registry → Full Dockerfile (use templates)
├─ Open source → Full Dockerfile + clear build instructions
└─ Unsure → Start with mounted code, add Dockerfile later

Auditing Existing Dockerfile

Priority order:
1. Security (non-root, no secrets) → Fix immediately
2. Base image (slim, pinned) → Quick win
3. .dockerignore → 5 minutes, big impact
4. Multi-stage → If image > 500MB
5. Cache mounts → If builds > 2min

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Version Info

• Last updated: December 2025
• Docker version: 27.x+ (BuildKit default)
• Philosophy: Start simple, optimize when needed
• Sources: Docker docs, OWASP, Google Distroless

Source: DigitalPine/claude-skills — distributed by TomeVault.