What is the difference between Docker containers and VMs? What is the relationship between Image and Container?

Containers vs Virtual Machines

Characteristic	Virtual Machine	Container
Isolation level	OS-level	Process-level
Startup time	Minutes	Seconds
Size	GBs	MBs
Resource overhead	High (each VM has full OS)	Low (shares host OS kernel)
Portability	Lower	High

Image vs Container

Image: A read-only filesystem snapshot containing the application, dependencies, and configuration. It is a static template.

Container: A running instance of an image, with an added writable layer. It is a dynamic runtime state.

Analogy: An image is like source code, a container is like a running process.

Basic Dockerfile Structure

FROM node:18-alpine WORKDIR /app COPY package*.json ./ RUN npm ci --only=production COPY . . EXPOSE 3000 CMD ["node", "server.js"]

Layer Caching

Each Dockerfile instruction creates a layer. Docker caches unchanged layers. Place frequently changing instructions (like COPY . .) at the end so static dependency layers can be reused from cache, speeding up builds.