docker

Skill: docker

Docker

Overview

Containerization, Dockerfile architecture, security best practices, and container orchestration for production workloads.

When to Use

Activate when creating or optimizing Dockerfiles, docker-compose configurations, container security audits, or CI/CD container builds.

Rules & Patterns

Negative Constraints (What NOT to Do)

1. NEVER run containers as root in production: Always create and switch to an unprivileged non-root user (e.g. USER node or USER nonroot).
2. NEVER use the latest tag: Always pin base images to specific immutable version digests or explicit minor tags (e.g. node:20.12.2-alpine3.19).
3. NEVER copy source code before package.json: Always copy lockfiles and install dependencies first to leverage Docker's layer caching.
4. NEVER bake secrets, API keys, or .env files into image layers: Pass secrets via build-time secret mounts (--mount=type=secret) or runtime environment variables.
5. NEVER include build tools or devDependencies in the final runner image: Always use multi-stage builds to discard compilers and package managers from production images.

Multi-Stage Standard Pattern

FROM node:20.12.2-alpine3.19 AS builder
WORKDIR /app
COPY package.json package-lock.json ./
RUN npm ci
COPY . .
RUN npm run build && npm prune --production

FROM node:20.12.2-alpine3.19 AS runner
WORKDIR /app
ENV NODE_ENV=production
RUN addgroup -S -g 1001 appgroup && adduser -S -u 1001 appuser -G appgroup
COPY --from=builder --chown=appuser:appgroup /app/dist ./dist
COPY --from=builder --chown=appuser:appgroup /app/node_modules ./node_modules
USER appuser
CMD ["node", "dist/index.js"]

Code Examples

See EXAMPLES.md for production Dockerfiles and dockerignore patterns.

Validation Checklist

• Multi-stage build separates build tools from runtime
• Non-root USER directive active in final stage
• Base images pinned to exact versions
• .dockerignore file prevents leaking node_modules or secrets