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

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