Dockerfile Instructions

# Official image
FROM ubuntu:22.04

# Alpine variant (smaller)
FROM node:20-alpine

# Specific digest (immutable)
FROM nginx@sha256:abc123...

# Platform specific
FROM --platform=linux/amd64 node:20

# Named stage for building
FROM node:20-alpine AS builder
WORKDIR /app
COPY . .
RUN npm run build

# Production stage
FROM nginx:alpine AS production
COPY --from=builder /app/dist /usr/share/nginx/html

# Empty base image (for static binaries)
FROM scratch
COPY myapp /myapp
CMD ["/myapp"]

# Define with default value
ARG NODE_VERSION=20
ARG APP_VERSION

# Use in instructions
FROM node:${NODE_VERSION}-alpine

# ARG after FROM needs redeclaration
ARG APP_VERSION
RUN echo "Building version ${APP_VERSION}"

# Build with arguments
docker build --build-arg APP_VERSION=1.0.0 -t myapp .

# Single variable
ENV NODE_ENV=production

# Multiple variables
ENV NODE_ENV=production \
    PORT=3000 \
    HOST=0.0.0.0

# Use in subsequent instructions
RUN echo "Environment: $NODE_ENV"

# ARG for build-time, ENV for runtime
ARG VERSION=latest
ENV APP_VERSION=${VERSION}

# Now APP_VERSION is available at runtime
# VERSION is only available during build

# Copy single file
COPY package.json .

# Copy multiple files
COPY package.json package-lock.json ./

# Copy directory
COPY src/ ./src/

# Copy with glob patterns
COPY *.json ./

# Copy from previous stage
COPY --from=builder /app/dist ./dist

# Copy with ownership
COPY --chown=node:node . .

# Auto-extract tar files
ADD archive.tar.gz /app/

# Download from URL (not recommended)
ADD https://example.com/file.txt /app/

# Same as COPY
ADD src/ ./src/

# Shell form (runs in /bin/sh -c)
RUN apt-get update && apt-get install -y curl

# Environment variable expansion works
RUN echo "Home is $HOME"

# Pipe support
RUN curl -sL https://example.com | tar xz

# Exec form (no shell processing)
RUN ["apt-get", "install", "-y", "curl"]

# No variable expansion
RUN ["echo", "$HOME"]  # Prints literal $HOME

# Different shell
RUN ["/bin/bash", "-c", "echo Hello"]

# ✅ Combine commands to reduce layers
RUN apt-get update && \
    apt-get install -y --no-install-recommends \
        curl \
        wget \
        git && \
    rm -rf /var/lib/apt/lists/*

# ✅ Clean up in the same layer
RUN npm install && \
    npm cache clean --force

# ❌ Don't split update and install
RUN apt-get update
RUN apt-get install -y curl  # May use stale cache

# Exec form (preferred)
CMD ["npm", "start"]

# Shell form
CMD npm start

# As ENTRYPOINT arguments
CMD ["--port", "3000"]

# CMD can be overridden
docker run myapp npm test  # Overrides CMD

# Exec form (preferred)
ENTRYPOINT ["node", "server.js"]

# Shell form (not recommended)
ENTRYPOINT node server.js

# ENTRYPOINT is always executed
docker run myapp  # Runs: node server.js
docker run myapp --port 3000  # Runs: node server.js --port 3000

# ENTRYPOINT + CMD pattern
ENTRYPOINT ["python", "app.py"]
CMD ["--host", "0.0.0.0"]

# Results in: python app.py --host 0.0.0.0

# Override CMD but keep ENTRYPOINT
docker run myapp --port 8080
# Runs: python app.py --port 8080

# Override ENTRYPOINT
docker run --entrypoint bash myapp
# Runs: bash