Bits of .NET

📚 Each Dockerfile Line = One Layer

Docker images are layers. Understanding layers helps optimize builds. Cache, reuse, smaller images.

📝 How Layers Work

Dockerfile:
FROM node:18           # Layer 1: Base image
WORKDIR /app           # Layer 2: Set working directory
COPY package*.json ./  # Layer 3: Copy package files
RUN npm install        # Layer 4: Install dependencies
COPY . .               # Layer 5: Copy source
RUN npm run build      # Layer 6: Build
CMD ["npm", "start"]  # Layer 7: Command

# Each layer is cached
# Changing layer 5 invalidates layers 5, 6, 7
# But layers 1-4 are reused (cached)

# View layers
docker history myapp:latest

# Optimize: Put frequently changed files LAST
COPY package*.json ./  # Changes rarely
RUN npm install        # Cached
COPY . .               # Changes often (last)

🎯 Layer Optimization Tips

# 1. Combine RUN commands
RUN apt-get update && apt-get install -y \
    git \
    curl \
    wget \
    && rm -rf /var/lib/apt/lists/*

# 2. Use multi-stage builds
FROM node:18 AS builder
COPY . .
RUN npm run build

FROM nginx:alpine
COPY --from=builder /app/dist /usr/share/nginx/html

# 3. Leverage cache
COPY package*.json ./
RUN npm install
COPY . .  # Changes here doesn't invalidate npm install layer

# 4. Use .dockerignore
# Exclude node_modules, .git, etc. from build context

# 5. Use smaller base images
# node:18-alpine (170MB) vs node:18 (950MB)

💡 Commands

• docker history image:tag (show layers)
• docker build –no-cache (rebuild without cache)
• docker image prune -a (remove unused layers)
• docker system df (check disk usage by layers)