Security (Critical)
- Run as non-root user (
USER directive) - Don't store secrets in image (use runtime injection)
- Don't use
--privileged without justification - Scan images for vulnerabilities
- Set
readonly root filesystem where possible - Review any use of build-time variables (e.g.,
ARG, ENV, LABEL values) that can be influenced by external inputs (such as --build-arg values or CI/CD environment variables sourced from untrusted users) to ensure they are not used in a way that enables build-time injection - Never use HTML comments (
Base Images
- Pin base image to specific version (not
latest) - Use official images from trusted sources
- Prefer minimal images (
alpine, slim, distroless) - Regularly update base images for security patches
Build Optimization
- Use multi-stage builds to reduce final image size
- Order instructions by change frequency (cache optimization)
- Combine
RUN commands to reduce layers - Use
.dockerignore to exclude unnecessary files, sensitive data, and build artifacts like node_modules
Instructions (Essential)
- Use
COPY instead of ADD (unless extracting archives) - Set
WORKDIR before COPY/RUN - Use explicit
EXPOSE for documentation - Set meaningful
LABEL metadata
Additional Instructions
- Explicitly set
SHELL if bash/sh features are needed - Set environment variables with
ENV for configuration (not secrets) - Clean up package manager caches after install (e.g.,
apt-get clean) - Understand
ENTRYPOINT vs CMD: use ENTRYPOINT for main command, CMD for default args - Document container usage with OCI labels (
org.opencontainers.image.*)
Health Checks
- Include
HEALTHCHECK instruction - Health check should verify app is actually working
- Set appropriate interval and timeout
Example Good Dockerfile Pattern
```dockerfile
# Build stage
FROM node:20-alpine AS builder
WORKDIR /app
COPY package*.json ./
RUN npm ci --only=production
# Runtime stage
FROM node:20-alpine
# Add OCI labels for documentation
LABEL org.opencontainers.image.title="My App"
LABEL org.opencontainers.image.description="Production web application"
LABEL org.opencontainers.image.version="1.0.0"
RUN addgroup -S appgroup && adduser -S appuser -G appgroup
WORKDIR /app
# Copy dependencies and app files
COPY --from=builder /app/node_modules ./node_modules
COPY . .
# Set environment variables (not secrets)
ENV NODE_ENV=production
USER appuser
EXPOSE 3000
HEALTHCHECK CMD wget -q --spider http://localhost:3000/health || exit 1
# Use ENTRYPOINT for main command, CMD for default args
ENTRYPOINT ["node"]
CMD ["server.js"]
```
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