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Writing Dockerfiles

A Dockerfile is a plain-text script of instructions that Docker executes top-to-bottom to build an image. Each instruction creates a new layer.

Dockerfile Instructions Reference​

FROM — Base Image​

FROM eclipse-temurin:21-jre-alpine

# Multi-stage: name each stage
FROM maven:3.9-eclipse-temurin-21 AS builder
FROM eclipse-temurin:21-jre-alpine AS runtime

Always pin to a specific version tag. Never use FROM ubuntu (resolves to latest).

WORKDIR — Set Working Directory​

WORKDIR /app
# Creates the directory if it doesn't exist.
# All subsequent instructions run relative to this path.
# Preferred over RUN cd /app

COPY vs ADD​

# COPY — preferred for most cases
COPY src/main/resources/application.yml /app/config/
COPY target/myapp.jar /app/myapp.jar
COPY . .   # Copy everything from build context

# ADD — avoid unless you specifically need its extras:
#   - Auto-extract tar archives (ADD app.tar.gz /app)
#   - Fetch remote URLs (avoid — non-deterministic, use curl instead)
ADD https://example.com/file.tar.gz /tmp/  # ❌ Non-reproducible

RUN — Execute Commands​

# Single command
RUN apt-get update

# Chain commands with && to keep in ONE layer
# ✅ Efficient — one layer
RUN apt-get update && \
    apt-get install -y --no-install-recommends curl wget && \
    rm -rf /var/lib/apt/lists/*   # Clean up in same layer!

# ❌ Inefficient — three layers
RUN apt-get update
RUN apt-get install -y curl
RUN rm -rf /var/lib/apt/lists/*

Clean up package manager caches in the same RUN instruction — once the layer is committed, you can't remove files from it in a later layer.

CMD vs ENTRYPOINT​

# CMD — default command, easily overridden at runtime
CMD ["java", "-jar", "/app/myapp.jar"]
docker run myapp                          # runs: java -jar /app/myapp.jar
docker run myapp java -jar other.jar      # overrides CMD entirely

# ENTRYPOINT — fixed command, arguments are appended
ENTRYPOINT ["java"]
CMD ["-jar", "/app/myapp.jar"]
docker run myapp                          # runs: java -jar /app/myapp.jar
docker run myapp -jar other.jar           # runs: java -jar other.jar (CMD overridden)

# Best pattern for applications — combine both:
ENTRYPOINT ["java"]
CMD ["-jar", "/app/myapp.jar"]

# Exec form (preferred) vs Shell form:
CMD ["java", "-jar", "app.jar"]   # ✅ Exec form — signals go directly to process
CMD java -jar app.jar             # ❌ Shell form — runs via /bin/sh -c, PID 1 is shell

ENV — Environment Variables​

ENV APP_HOME=/app
ENV JAVA_OPTS="-Xms256m -Xmx512m"
ENV SPRING_PROFILES_ACTIVE=production

# Multi-line (Docker 1.9+)
ENV APP_HOME=/app \
    APP_PORT=8080

# Access in RUN
RUN echo $APP_HOME

Prefer passing secrets at runtime (docker run -e SECRET=...) — ENV values baked into image are visible in image history.

ARG — Build-Time Variables​

ARG JAR_FILE=target/myapp.jar
ARG BUILD_DATE
ARG GIT_COMMIT

COPY ${JAR_FILE} /app/app.jar

# Pass at build time
docker build --build-arg JAR_FILE=target/myapp-1.0.jar \
             --build-arg GIT_COMMIT=$(git rev-parse HEAD) .

# ARG values not present in final image (unlike ENV)
# ⚠ Exception: ARG before FROM applies to FROM only
ARG BASE_IMAGE=eclipse-temurin:21-jre-alpine
FROM ${BASE_IMAGE}

EXPOSE — Document Port​

EXPOSE 8080
EXPOSE 8080/tcp
EXPOSE 9090/udp

# EXPOSE is documentation only — it does NOT publish the port.
# Use -p flag at runtime to actually publish:
docker run -p 8080:8080 myapp

VOLUME — Declare Mount Point​

VOLUME ["/app/data"]
VOLUME /app/logs /app/uploads

# Declares that these paths should be persisted outside the container.
# Docker creates an anonymous volume automatically at runtime.
# Better: explicitly define volumes in docker run / docker-compose.

USER — Non-Root User​

# Create group and user
RUN addgroup -S appgroup && adduser -S appuser -G appgroup
# Switch to non-root
USER appuser

# Or by UID
USER 1001

HEALTHCHECK — Container Health​

HEALTHCHECK --interval=30s --timeout=5s --start-period=30s --retries=3 \
  CMD curl -f http://localhost:8080/actuator/health || exit 1

# Status: healthy / unhealthy / starting

LABEL — Metadata​

LABEL maintainer="team@example.com" \
      version="1.0.0" \
      description="My Spring Boot API" \
      org.opencontainers.image.source="https://github.com/org/repo" \
      org.opencontainers.image.revision="${GIT_COMMIT}"

Multi-Stage Builds​

Build in one stage, copy only the result to the final image. Drastically reduces image size.

Spring Boot — Multi-Stage Build​

# ─── Stage 1: Build ─────────────────────────────────────────────
FROM maven:3.9-eclipse-temurin-21 AS builder
WORKDIR /build

# Copy POM first — layer cache: only re-download deps when pom.xml changes
COPY pom.xml .
RUN mvn dependency:go-offline -q

# Now copy source and build
COPY src ./src
RUN mvn package -DskipTests -q

# ─── Stage 2: Extract layers (Spring Boot layered JARs) ──────────
FROM builder AS extractor
WORKDIR /build
RUN java -Djarmode=layertools -jar target/*.jar extract --destination extracted

# ─── Stage 3: Runtime ────────────────────────────────────────────
FROM eclipse-temurin:21-jre-alpine AS runtime
WORKDIR /app

# Security: non-root user
RUN addgroup -S spring && adduser -S spring -G spring

# Copy Spring Boot layers (ordered by change frequency — slowest changing first)
COPY --from=extractor --chown=spring:spring /build/extracted/dependencies/ ./
COPY --from=extractor --chown=spring:spring /build/extracted/spring-boot-loader/ ./
COPY --from=extractor --chown=spring:spring /build/extracted/snapshot-dependencies/ ./
COPY --from=extractor --chown=spring:spring /build/extracted/application/ ./

USER spring

EXPOSE 8080

HEALTHCHECK --interval=30s --timeout=3s --start-period=40s --retries=3 \
  CMD wget -qO- http://localhost:8080/actuator/health || exit 1

ENTRYPOINT ["java", "org.springframework.boot.loader.launch.JarLauncher"]
Without multi-stage:   ~650 MB (JDK + Maven + source + test deps + JAR)
With multi-stage:      ~180 MB (JRE only + app layers)

Why Spring Boot Layers?​

Spring Boot layered JAR extracts into:
  dependencies/          ← third-party JARs — rarely change
  spring-boot-loader/    ← boot loader — rarely changes
  snapshot-dependencies/ ← your SNAPSHOT deps — change occasionally
  application/           ← your code — changes often

→ Only the "application" layer is rebuilt on code change
→ CI pipeline rebuilds only the changed layer (seconds, not minutes)

Layer Caching Optimization​

The #1 performance tip: put things that change rarely at the top, things that change often at the bottom.

# ❌ BAD ORDER — source code change invalidates ALL layers below
FROM eclipse-temurin:21-jre-alpine
COPY . .                          # ← Copies everything including source
RUN mvn package                   # ← Must re-run every time
COPY target/app.jar /app/app.jar

# ✅ GOOD ORDER — cache pom.xml deps independently of source
FROM maven:3.9-eclipse-temurin-21 AS builder
COPY pom.xml .                   # ← Only changes when deps change
RUN mvn dependency:go-offline    # ← Cached until pom.xml changes
COPY src ./src                   # ← Changes every commit
RUN mvn package -DskipTests      # ← Only re-runs when src changes

.dockerignore​

Exclude files from the build context sent to the Docker daemon. Speeds up builds and prevents secrets leaking into images.

# .dockerignore
.git
.gitignore
.idea
*.iml
target/             # Maven output — we build inside Docker
!target/myapp.jar   # But include final JAR if pre-built
*.log
.env                # ← NEVER send .env to Docker daemon
node_modules/
README.md
Dockerfile*
docker-compose*.yml

Security Hardening Checklist​

# ✅ 1. Use minimal base image
FROM eclipse-temurin:21-jre-alpine  # NOT full JDK, NOT Debian

# ✅ 2. Non-root user
RUN addgroup -S app && adduser -S app -G app
USER app

# ✅ 3. No secrets in image
# Never: ENV DB_PASSWORD=secret123
# Do: docker run -e DB_PASSWORD=$SECRET at runtime

# ✅ 4. Read-only filesystem (set at runtime or in K8s)
# docker run --read-only --tmpfs /tmp myapp

# ✅ 5. Minimal layers — no build tools in final image (multi-stage)

# ✅ 6. Pin exact versions
FROM eclipse-temurin:21.0.5_11-jre-alpine  # pinned patch version
# NOT: FROM eclipse-temurin:21

# ✅ 7. Scan image for vulnerabilities
# trivy image myapp:1.0.0

Common Dockerfile Patterns​

Pattern: Overridable JVM options​

ENV JAVA_OPTS=""
ENTRYPOINT ["sh", "-c", "java $JAVA_OPTS -jar /app/app.jar"]

# Override at runtime:
docker run -e JAVA_OPTS="-Xmx2g -XX:+UseG1GC" myapp

Pattern: Wait for dependencies​

# Use wait-for-it.sh or dockerize to wait for DB before starting app
COPY wait-for-it.sh /wait-for-it.sh
RUN chmod +x /wait-for-it.sh
CMD ["/wait-for-it.sh", "db:5432", "--", "java", "-jar", "/app/app.jar"]

Pattern: Configuration via environment​

# Spring Boot reads SPRING_* env vars automatically
docker run \
  -e SPRING_DATASOURCE_URL=jdbc:postgresql://db:5432/mydb \
  -e SPRING_DATASOURCE_USERNAME=user \
  -e SPRING_DATASOURCE_PASSWORD=$DB_PASS \
  -e SPRING_PROFILES_ACTIVE=production \
  myapp:1.0.0

Build Commands​

# Build an image
docker build -t myapp:1.0.0 .
docker build -t myapp:1.0.0 -f Dockerfile.prod .   # Custom Dockerfile name
docker build -t myapp:1.0.0 --no-cache .            # Force full rebuild
docker build -t myapp:1.0.0 --build-arg PROFILE=prod .

# Build and tag multiple
docker build -t myapp:1.0.0 -t myapp:latest .

# Multi-platform build (build for linux/amd64 AND linux/arm64)
docker buildx build --platform linux/amd64,linux/arm64 -t myapp:1.0.0 --push .

# Check image layers and size
docker history myapp:1.0.0
docker image inspect myapp:1.0.0

Interview Questions​

  1. What is the difference between CMD and ENTRYPOINT?
  2. What is a multi-stage build and why is it used?
  3. Why does instruction order matter in a Dockerfile?
  4. What is the .dockerignore file and why is it important?
  5. What is the difference between COPY and ADD?
  6. Why should you use RUN apt-get update && apt-get install in one RUN instruction?
  7. How do Spring Boot layered JARs improve Docker build performance?
  8. Why should containers run as a non-root user?
  9. What is the difference between ARG and ENV?
  10. What does EXPOSE actually do?