DOCKERFILE Dockerfile

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File Description

Dockerfile

What is a Dockerfile?

A Dockerfile is a text file that contains a series of instructions used to build a Docker image automatically. It serves as a blueprint for creating containerized applications by defining the environment, dependencies, configuration, and commands needed to run an application. Dockerfiles use a simple, declarative syntax that allows developers to version control their infrastructure and ensure consistent deployments across different environments.

More Information

Docker was introduced by Solomon Hykes at dotCloud (later Docker Inc.) in 2013, revolutionizing application deployment and development workflows. The Dockerfile concept was designed to provide a reproducible and automated way to build container images, eliminating the "it works on my machine" problem. Dockerfiles enable Infrastructure as Code (IaC) practices, allowing teams to define their application's runtime environment alongside their source code.

The Docker ecosystem has become fundamental to modern software development, particularly in microservices architectures, continuous integration/continuous deployment (CI/CD) pipelines, and cloud-native applications. Dockerfiles have standardized how applications are packaged and deployed, making it easier to scale applications, manage dependencies, and maintain consistency across development, testing, and production environments.

Dockerfile Format

Dockerfiles use a specific instruction-based syntax:

Basic Instructions

FROM - Specifies the base image
COPY/ADD - Copies files from host to container
RUN - Executes commands during image build
CMD - Default command to run when container starts
ENTRYPOINT - Configures container to run as executable
EXPOSE - Documents which ports the container listens on
ENV - Sets environment variables
WORKDIR - Sets working directory for subsequent instructions

Advanced Instructions

ARG - Build-time variables
LABEL - Adds metadata to images
USER - Sets user for subsequent instructions
VOLUME - Creates mount points for external volumes
ONBUILD - Triggers for derived images
HEALTHCHECK - Health monitoring instructions
SHELL - Changes default shell for RUN commands

Best Practices

Layer optimization - Minimize layers and image size
Cache efficiency - Order instructions for better caching
Security - Use non-root users, minimal base images
Multi-stage builds - Reduce final image size
Dependency management - Pin versions for reproducibility

Example Dockerfile

# Multi-stage build for Node.js application
FROM node:18-alpine AS builder

# Set working directory
WORKDIR /app

# Copy package files first (for better caching)
COPY package*.json ./

# Install dependencies
RUN npm ci --only=production

# Copy source code
COPY . .

# Build the application
RUN npm run build

# Production stage
FROM node:18-alpine AS production

# Create non-root user
RUN addgroup -g 1001 -S nodejs && \
    adduser -S nextjs -u 1001

# Set working directory
WORKDIR /app

# Copy built application from builder stage
COPY --from=builder --chown=nextjs:nodejs /app/dist ./dist
COPY --from=builder --chown=nextjs:nodejs /app/node_modules ./node_modules
COPY --from=builder --chown=nextjs:nodejs /app/package.json ./package.json

# Switch to non-root user
USER nextjs

# Expose port
EXPOSE 3000

# Health check
HEALTHCHECK --interval=30s --timeout=3s --start-period=5s --retries=3 \
  CMD curl -f http://localhost:3000/health || exit 1

# Start the application
CMD ["node", "dist/server.js"]