docker-best-practices

name	docker-best-practices
category	Backend
description	MUST USE when writing or editing Dockerfiles, docker-compose.yml, .dockerignore, or container configuration. Enforces multi-stage builds, layer caching, security hardening, Compose Watch for local dev, and health checks.

Docker Best Practices

Multi-Stage Builds

Separate build dependencies from runtime. Ship only what you need.

# BAD: build tools, devDependencies, and source all ship to production
FROM node:20
WORKDIR /app
COPY . .
RUN npm ci && npm run build
CMD ["node", "dist/index.js"]

# GOOD: multi-stage — clean production image
FROM node:20-slim AS build
WORKDIR /app
COPY package*.json ./
RUN npm ci
COPY . .
RUN npm run build

FROM node:20-slim
WORKDIR /app
COPY --from=build /app/dist ./dist
COPY --from=build /app/node_modules ./node_modules
COPY --from=build /app/package.json ./
USER node
CMD ["node", "dist/index.js"]

# GOOD: Go with distroless — ~2 MiB final image
FROM golang:1.22 AS build
WORKDIR /src
COPY go.mod go.sum ./
RUN go mod download
COPY . .
RUN CGO_ENABLED=0 go build -o /app ./cmd/server

FROM gcr.io/distroless/static-debian12
COPY --from=build /app /app
USER nonroot:nonroot
ENTRYPOINT ["/app"]

Layer Caching

Order instructions from least-frequently changed to most-frequently changed.

# BAD: every code change reinstalls dependencies
COPY . .
RUN pip install -r requirements.txt

# GOOD: deps cached unless requirements.txt changes
COPY requirements.txt .
RUN pip install --no-cache-dir -r requirements.txt
COPY . .

# BAD: separate RUN commands — extra layers, stale apt cache
RUN apt-get update
RUN apt-get install -y curl git

# GOOD: single layer, sorted packages, cache cleaned
RUN apt-get update && apt-get install -y --no-install-recommends \
    curl \
    git \
    && rm -rf /var/lib/apt/lists/*

Security — Non-Root User

# BAD: runs as root
FROM node:20-slim
WORKDIR /app
COPY . .
CMD ["node", "server.js"]

# GOOD: non-root user
FROM node:20-slim
WORKDIR /app
COPY --chown=node:node package*.json ./
RUN npm ci --omit=dev
COPY --chown=node:node . .
USER node
CMD ["node", "server.js"]

COPY --chown, Not RUN chown

Docker's copy-on-write stores files in two layers if you chown after COPY.

# BAD: files duplicated across two layers — doubles size
COPY . /app
RUN chown -R app:app /app

# GOOD: ownership set during copy — single layer
COPY --chown=app:app . /app

# BAD                              # GOOD
COPY entrypoint.sh /entrypoint.sh  COPY --chmod=755 entrypoint.sh /entrypoint.sh
RUN chmod +x /entrypoint.sh

Build Secrets

ARG and ENV persist in image layers. Use --mount=type=secret.

# BAD: secret baked into image (visible via docker history)
ARG NPM_TOKEN
RUN echo "//registry.npmjs.org/:_authToken=${NPM_TOKEN}" > .npmrc && \
    npm ci && rm .npmrc  # too late — it's in a previous layer

# GOOD: secret mounted temporarily, never persisted
RUN --mount=type=secret,id=npmrc,target=/root/.npmrc npm ci
# Build: docker build --secret id=npmrc,src=$HOME/.npmrc .

CMD — Exec Form

# BAD: shell form — PID 1 is /bin/sh, app doesn't receive SIGTERM
CMD npm start

# GOOD: exec form — app is PID 1, receives signals properly
CMD ["node", "server.js"]

Base Images — Pin Versions

# BAD: non-reproducible
FROM node:latest

# GOOD: pinned slim image
FROM node:20.11-slim

Use <lang>-slim for most apps. Use distroless/static or scratch for static binaries (Go, Rust).

.dockerignore

Always create one. Without it, .git, node_modules, .env enter the build context.

.git
node_modules
dist
*.md
.env*
.vscode
__pycache__
*.pyc
docker-compose*.yml
Dockerfile*

Health Checks

# In Compose — use with depends_on for startup ordering
services:
  db:
    image: postgres:16
    healthcheck:
      test: ["CMD-SHELL", "pg_isready -U postgres"]
      interval: 5s
      timeout: 5s
      retries: 5

  api:
    build: .
    depends_on:
      db:
        condition: service_healthy
    healthcheck:
      test: ["CMD", "curl", "-f", "http://localhost:8000/health"]
      interval: 30s
      timeout: 5s
      retries: 3
      start_period: 10s

Compose Watch — Local Dev

Replaces bind mount hacks. One-way sync from host to container: sync (hot reload), sync+restart (config changes), rebuild (dependency changes).

# BAD: bidirectional bind mount — can clobber node_modules
services:
  web:
    build: .
    volumes:
      - .:/app
      - /app/node_modules  # anonymous volume hack

# GOOD: Compose Watch with granular rules
services:
  web:
    build: .
    command: npm run dev
    develop:
      watch:
        - action: sync
          path: ./src
          target: /app/src
          ignore:
            - node_modules/
        - action: sync+restart
          path: ./config
          target: /app/config
        - action: rebuild
          path: package.json

Start with: docker compose watch

Full Local Dev Stack

services:
  api:
    build: .
    ports:
      - "8000:8000"
    env_file: .env
    depends_on:
      db:
        condition: service_healthy
      redis:
        condition: service_healthy
    develop:
      watch:
        - action: sync
          path: ./src
          target: /app/src
        - action: rebuild
          path: requirements.txt

  db:
    image: postgres:16
    volumes:
      - pgdata:/var/lib/postgresql/data
    environment:
      POSTGRES_DB: app
      POSTGRES_USER: app
      POSTGRES_PASSWORD: localdev
    healthcheck:
      test: ["CMD-SHELL", "pg_isready -U app"]
      interval: 5s
      retries: 5

  redis:
    image: redis:7-alpine
    healthcheck:
      test: ["CMD", "redis-cli", "ping"]
      interval: 5s
      retries: 5

volumes:
  pgdata:

Rules

1. Always use multi-stage builds — separate build from runtime
2. Copy dependency files before source code — maximize layer caching
3. Never run as root — add USER directive in every Dockerfile
4. Use COPY --chown — never COPY then RUN chown
5. Never bake secrets into layers — use --mount=type=secret
6. Use exec form for CMD and ENTRYPOINT — ["binary", "arg"]
7. Pin base image versions — never use latest
8. Always create .dockerignore — exclude .git, node_modules, .env
9. Add health checks — use with depends_on: condition: service_healthy
10. Use Compose Watch for local dev — not bind mount volume hacks