18_DockerFileWritting

# =========================================
# Dockerfile Cheat Sheet (Complete – SDE-1)
# =========================================


# ---------- BASE IMAGE ----------
# FROM sets the base image for your application
# Everything runs on top of this image
FROM node:20-alpine


# ---------- BUILD ARGUMENT ----------
# ARG is used at BUILD TIME only
# Can be passed using: docker build --build-arg NODE_ENV=dev .
ARG NODE_ENV=production


# ---------- ENVIRONMENT VARIABLES ----------
# ENV is available at RUNTIME inside container
ENV NODE_ENV=${NODE_ENV}
ENV PORT=3000


# ---------- WORKING DIRECTORY ----------
# Sets the working directory inside container
# If not exists, Docker creates it
# All COPY, RUN, CMD work relative to this
WORKDIR /app


# ---------- COPY (Dependency Files First) ----------
# Copy only dependency files
# Helps Docker layer caching
COPY package.json package-lock.json ./


# ---------- RUN (Build-time command) ----------
# Executes during image build
# Each RUN creates a new image layer
RUN npm install


# ---------- COPY (Application Code) ----------
# Copies entire project from build context
# First '.' → host build context
# Second '.' → WORKDIR (/app)
COPY . .


# ---------- USER (Security Best Practice) ----------
# Avoid running app as root (optional for SDE-1)
# RUN addgroup -S app && adduser -S app -G app
# USER app


# ---------- EXPOSE ----------
# Documents the port the app listens on
# Does NOT actually open the port
EXPOSE 3000


# ---------- HEALTHCHECK ----------
# Lets Docker know how to check if app is healthy
# Optional but good to know
HEALTHCHECK --interval=30s --timeout=5s \
  CMD curl -f http://localhost:3000/ || exit 1


# ---------- ENTRYPOINT ----------
# Defines the main executable
# Usually fixed and not overridden
# ENTRYPOINT ["node"]


# ---------- CMD ----------
# Default command when container starts
# Can be overridden at runtime
CMD ["npm", "start"]


# =========================================
# KEY CONCEPT NOTES (Interview Gold)
# =========================================
# RUN        → build time
# CMD        → runtime
# ENTRYPOINT → fixed executable
# ARG        → build time only
# ENV        → runtime environment
# COPY       → host → container
# WORKDIR   → current directory inside container
# EXPOSE    → documentation only
# =========================================



# --------------------------------------------Mulristage build-------------------------------------------------------------
    # =========================================
# MULTI-STAGE DOCKERFILE (Node.js Example)
# =========================================


# ---------- STAGE 1: BUILD STAGE ----------
# This stage is used ONLY to build the app
# It can be large and heavy (dev dependencies allowed)

FROM node:20 AS builder
# AS builder → names this stage "builder"

# Set working directory inside container
WORKDIR /app

# Copy dependency files first for caching
COPY package.json package-lock.json ./

# Install ALL dependencies (including dev)
RUN npm install

# Copy application source code
COPY . .

# Build the application (e.g. React / TS / Next build)
RUN npm run build


# ---------- STAGE 2: PRODUCTION STAGE ----------
# This stage is the FINAL image
# Should be small, secure, production-ready

FROM node:20-alpine
# Smaller base image for production

# Set working directory
WORKDIR /app

# Copy only the built output from builder stage
# --from=builder → copy files from stage 1
COPY --from=builder /app/dist ./dist

# Copy only required files
COPY package.json package-lock.json ./

# Install ONLY production dependencies
RUN npm install --production

# Expose application port
EXPOSE 3000

# Start the app
CMD ["node", "dist/index.js"]


# =========================================
# WHY MULTI-STAGE?
# - Build tools stay in builder image
# - Final image is smaller & secure
# - Faster deployments
# =========================================

# Why we need Multi-Stage Docker builds (exactly) Core idea (1 line 🔥)
# Multi-stage builds separate build-time work from runtime needs, so the final image contains only what is required to run the app.





# Advices
# 1. dockerignore file is also there that docker cant see
# 2. volume mostly good to use in compose file