# Docker Best Practices

> Docker/container best practices — multi-stage builds, minimal base images, non-root users, .dockerignore, layer caching, secrets management, health checks, docker-compose patterns

- Skill: `nadav011/docker-best-practices` (Agent Skill)
- Install (CLI): `npx skillmds@latest add nadav011/docker-best-practices`
- Raw SKILL.md: https://api.skillmd.com/api/skills/nadav011/docker-best-practices/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: DevOps & Infra
- Author: Nadav011 (https://skillmd.com/u/nadav011)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/nadav011/docker-best-practices

---

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# Docker Best Practices

---

## Multi-Stage Builds

Build artifacts in a full image; copy only the runtime output into a minimal final image. This removes build tools, source code, and dev dependencies from production containers.

```dockerfile
# ❌ WRONG — single stage: build tools, source, and node_modules all end up in the image
FROM node:20
WORKDIR /app
COPY . .
RUN npm install
RUN npm run build
CMD ["node", "dist/server.js"]
# Final image: ~1.2 GB
```

```dockerfile
# ✅ CORRECT — multi-stage: final image contains only the compiled output
FROM node:20-slim AS builder
WORKDIR /app
COPY package*.json ./
RUN npm ci --ignore-scripts
COPY . .
RUN npm run build

FROM node:20-slim AS runner
WORKDIR /app
ENV NODE_ENV=production
COPY package*.json ./
RUN npm ci --omit=dev --ignore-scripts
COPY --from=builder /app/dist ./dist
CMD ["node", "dist/server.js"]
# Final image: ~180 MB
```

---

## Minimal Base Images

Choose the smallest image that satisfies your runtime requirements.

| Base | Typical Size | Use When |
|------|-------------|----------|
| `ubuntu:24.04` | ~80 MB | You need apt packages and broad compatibility |
| `node:20-slim` | ~80 MB | Node apps that need a few system libs |
| `node:20-alpine` | ~45 MB | Node apps with no native add-ons |
| `gcr.io/distroless/nodejs20` | ~20 MB | Production Node — no shell, no package manager |

```dockerfile
# ❌ WRONG — full Ubuntu for a compiled Go binary: 900 MB image
FROM ubuntu:24.04
COPY myapp /usr/local/bin/myapp
CMD ["myapp"]

# ✅ CORRECT — scratch for a statically linked binary: ~10 MB image
FROM scratch
COPY myapp /myapp
CMD ["/myapp"]

# ✅ CORRECT — distroless for a Node.js app
FROM gcr.io/distroless/nodejs20-debian12
COPY --from=builder /app/dist /app/dist
COPY --from=builder /app/node_modules /app/node_modules
CMD ["/app/dist/server.js"]
```

---

## Non-Root User

Containers run as root by default. If the application is compromised, the attacker has root inside the container — and potentially a path to the host.

```dockerfile
# ❌ WRONG — running as root
FROM node:20-slim
WORKDIR /app
COPY --from=builder /app/dist ./dist
CMD ["node", "dist/server.js"]
# If exploited: attacker is root in container

# ✅ CORRECT — create and switch to a dedicated user
FROM node:20-slim
WORKDIR /app
RUN addgroup --system --gid 1001 nodejs \
 && adduser --system --uid 1001 --ingroup nodejs appuser
COPY --chown=appuser:nodejs --from=builder /app/dist ./dist
COPY --chown=appuser:nodejs --from=builder /app/node_modules ./node_modules
USER appuser
CMD ["node", "dist/server.js"]
```

---

## .dockerignore

Without `.dockerignore`, every file in your project directory is sent to the Docker build context — including `.env` files, `node_modules`, `.git` history, and test fixtures.

```
# .dockerignore
.git
.gitignore
node_modules
npm-debug.log
.env
.env.*
*.local
dist
coverage
.nyc_output
.next
.turbo
Dockerfile*
docker-compose*.yml
README.md
tests
__tests__
*.test.ts
*.spec.ts
```

**Why it matters:**
- Build context size directly affects `docker build` time and remote build transfer size
- `.env` files containing secrets must never reach the image layer, even if not `COPY`d — they're in the build context and can be accessed by malicious build steps

---

## Layer Caching Optimization

Docker rebuilds every layer after the first changed layer. Copy dependency manifests first, install, then copy source.

```dockerfile
# ❌ WRONG — source copied before npm install; any code change invalidates the cache
FROM node:20-slim
WORKDIR /app
COPY . .          # ← cache miss on any file change
RUN npm ci
RUN npm run build

# ✅ CORRECT — dependency install cached separately from source
FROM node:20-slim AS builder
WORKDIR /app
COPY package.json package-lock.json ./  # ← cache miss only when deps change
RUN npm ci
COPY . .                                # ← source copied after deps are installed
RUN npm run build
```

**Rule:** Order Dockerfile instructions from least-frequently-changed to most-frequently-changed.

---

## Secrets Management: Never Use ENV for Secrets

`ENV` values are baked into image layers and visible with `docker inspect`. Anyone who can pull the image can read the secrets.

```dockerfile
# ❌ WRONG — secret visible in image history and docker inspect
# Setting DATABASE_URL or API_KEY as ENV bakes it into every image layer.
# Anyone who runs `docker inspect` or `docker history` can extract the value.

# ❌ WRONG — build ARG also leaks into image metadata
ARG API_KEY
RUN curl -H "Authorization: $API_KEY" https://api.example.com/config > config.json
```

```dockerfile
# ✅ CORRECT — mount secret at build time (BuildKit); never stored in any layer
# syntax=docker/dockerfile:1
FROM node:20-slim AS builder
RUN --mount=type=secret,id=npm_token \
    npm config set //registry.npmjs.org/:_authToken=$(cat /run/secrets/npm_token) \
 && npm ci

# ✅ CORRECT — inject secrets at runtime via environment, not baked into image
# docker run --env-file .env myapp
# or in docker-compose: env_file: [.env]
```

```yaml
# docker-compose.yml — runtime env injection, not baked into the image
services:
  api:
    image: myapp:latest
    env_file:
      - .env          # loaded at container start, never written into an image layer
```

---

## Health Checks

Without a `HEALTHCHECK`, orchestrators (Kubernetes, ECS, Compose) can't tell if your container is actually ready to serve traffic — they just check that the process is running.

```dockerfile
# ❌ WRONG — no health check; orchestrator routes traffic immediately on process start
FROM node:20-slim
COPY --from=builder /app/dist ./dist
CMD ["node", "dist/server.js"]

# ✅ CORRECT — HTTP health check with sensible retries
FROM node:20-slim
COPY --from=builder /app/dist ./dist
HEALTHCHECK --interval=30s --timeout=5s --start-period=15s --retries=3 \
  CMD curl -f http://localhost:3000/health || exit 1
CMD ["node", "dist/server.js"]
```

```typescript
// Minimal /health endpoint in your app
app.get('/health', (_req, res) => {
  res.status(200).json({ status: 'ok', uptime: process.uptime() });
});
```

---

## docker-compose Patterns

```yaml
# ✅ CORRECT — production-safe docker-compose.yml
services:
  api:
    build:
      context: .
      target: runner           # stop at the 'runner' stage in multi-stage build
    image: myapp:${TAG:-latest}
    restart: unless-stopped
    ports:
      - "3000:3000"
    env_file:
      - .env                   # secrets injected at runtime
    depends_on:
      db:
        condition: service_healthy
    healthcheck:
      test: ["CMD", "curl", "-f", "http://localhost:3000/health"]
      interval: 30s
      timeout: 5s
      retries: 3

  db:
    image: postgres:16-alpine
    restart: unless-stopped
    environment:
      POSTGRES_DB: myapp
      POSTGRES_USER: myapp
    env_file:
      - .env                   # POSTGRES_PASSWORD sourced from .env — not inline
    volumes:
      - postgres_data:/var/lib/postgresql/data
    healthcheck:
      test: ["CMD-SHELL", "pg_isready -U myapp"]
      interval: 10s
      timeout: 5s
      retries: 5

volumes:
  postgres_data:
```

---

## Checklist

- [ ] Multi-stage build used — no build tools or source in the final image
- [ ] Minimal base image chosen for the runtime (slim, alpine, or distroless)
- [ ] Non-root user created and set with `USER` directive
- [ ] `.dockerignore` present — excludes `.env`, `node_modules`, `.git`, test files
- [ ] `package*.json` copied and deps installed before source is copied (cache optimization)
- [ ] No secrets in `ENV` or `ARG` instructions — runtime injection via env_file or secrets mount
- [ ] `HEALTHCHECK` defined with interval, timeout, and retries
- [ ] `depends_on` uses `condition: service_healthy` when ordering matters
- [ ] Image tagged explicitly — no bare `latest` tag in production deployments
- [ ] `restart: unless-stopped` set on long-running services