How Docker Builds Images

Docker builds images by executing each Dockerfile instruction sequentially, creating an immutable layer for each "RUN", "COPY", or "ADD" command. These layers stack on top of each other, forming the final image. Docker caches each layer and reuses it in subsequent builds if the instruction and all previous layers remain unchanged. This means the first instruction that changes invalidates its cache and all layers after it, forcing those instructions to re-execute.

The Cache Invalidation Problem

Most teams place "COPY . ." early in their Dockerfile, meaning every code change triggers a full rebuild including dependency installation. This turned routine deployments into 8-minute waits because Docker couldn't distinguish between application code changes and dependency changes, forcing complete reinstallation of packages that hadn't actually changed.

Strategic Instruction Ordering

The solution lies in separating dependency installation from application code. Place "COPY package.json" and "RUN npm install" before "COPY . .", so dependency layers only rebuild when package.json changes.

Optimized Layer Order

FROM node:18.16-alpine
WORKDIR /app
COPY package.json package-lock.json ./
RUN npm ci --only=production
COPY . .
CMD ["node", "server.js"]

This ordering follows the "least to most frequently changed" pattern: base image, system packages, language runtime, dependencies, configuration, then source code. With typical development workflows where application code changes but dependencies don't, 90% of builds reuse 80% of cached layers.

Multi-Stage Builds and Context Optimization

Multi-stage builds eliminate build tools from production images. Compile with full development dependencies in the first stage, then copy only the final artifacts into a minimal runtime image like "alpine" or "distroless". This reduced a 1.2GB Node.js image to 80MB, deploying 15x faster with a smaller attack surface.

The ".dockerignore" file prevents unnecessary files like ".git/", "node_modules/", and test directories from entering the build context. This cut context upload time from 30 seconds to 2 seconds and prevented cache invalidation when documentation changed.

5 Quick Tips for Speedy and Optimized Docker Images

1. Pin specific base image tags like "node:18.16-alpine" instead of "latest" to prevent unexpected cache invalidation.
2. Use BuildKit's "--mount=type=cache" for persistent package manager caches across builds, making "npm install" 5x faster.
3. Combine multiple "RUN" commands with "&&" to reduce layer count.
4. In CI/CD pipelines without shared local cache, leverage distributed caching with Google Cloud Build's kaniko executor or "docker buildx".
5. Always create a ".dockerignore" file to exclude unnecessary files from the build context, preventing cache invalidation from irrelevant file changes.