Docker Explained: Containerization, Namespaces, Cgroups & Multi-Stage Builds
Comprehensive Docker guide: kernel namespaces, cgroups, layered image filesystems, writing optimized multi-stage Dockerfiles for Java and Node.
# Multi-stage Dockerfile for Spring Boot Application
# Stage 1: Build JAR using Maven
FROM maven:3.9-eclipse-temurin-21-alpine AS builder
WORKDIR /app
COPY pom.xml .
RUN mvn dependency:go-offline
COPY src ./src
RUN mvn clean package -DskipTests
# Stage 2: Minimal Production JRE Image
FROM eclipse-temurin:21-jre-alpine
WORKDIR /app
COPY --from=builder /app/target/*.jar app.jar
EXPOSE 8080
USER 1000:1000
ENTRYPOINT ["java", "-jar", "app.jar"]1. Containers vs. Virtual Machines
A Virtual Machine runs a full guest operating system on top of a hypervisor, consuming gigabytes of memory and requiring minutes to boot. Docker containers share the host Linux kernel, isolating processes using two fundamental Linux kernel features:
- Namespaces: Isolate view of system resources (PID, NET, MNT, IPC, UTS).
- Control Groups (cgroups): Meter and limit hardware consumption (CPU, RAM, I/O bandwidth).
2. The Power of Multi-Stage Builds
Multi-stage builds allow developers to compile source code in a heavy build environment with compilers and SDKs, then copy only the compiled binary into a lightweight production runtime image, reducing image size from 800MB to under 120MB.
- Docker images use immutable, cached layers; order Dockerfile steps from least-frequently changed to most-frequently changed.
- Never run container processes as root; declare a dedicated non-root USER.
- Multi-stage builds dramatically shrink attack surface and download times.
Frequently Asked Questions
What is the difference between CMD and ENTRYPOINT?
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