Docker Fundamentals for Backend Developers

Dockerfile, Image and Container

Relationship

flowchart LR
    A[Dockerfile] -->|docker build| B[Docker Image]
    B -->|docker run| C[Container]

The full path from source code to a running container, for a Maven-based Spring Boot app:

flowchart LR
A[Source Code] --> B[Maven Build]
B --> C[JAR]
C --> D[Docker Image]
D --> E[Docker Container]
E --> F[Kubernetes / Server]

Component	Purpose	Analogy
Dockerfile	Instructions to build an image	Recipe
Docker Image	Packaged application and runtime	Packed meal
Container	Running instance of an image	Meal being eaten

Dockerfile

A Dockerfile is a text file containing instructions for building an image.

Node.js Example

FROM node:20

WORKDIR /usr/src/app

COPY package*.json ./

RUN npm install

COPY . .

EXPOSE 3000

CMD ["npx", "nodemon", "app.js"]

Spring Boot Example

FROM eclipse-temurin:17-jdk-jammy

WORKDIR /app

COPY target/myapp.jar app.jar

EXPOSE 8080

ENTRYPOINT ["java","-jar","app.jar"]

Building and Running Images

Build:

docker build -t node-app .

Run:

docker run -d --name node-app -p 3000:3000 node-app

Logs:

docker logs node-app

Stop:

docker stop node-app

Docker Compose

Compose is used to orchestrate multiple containers.

flowchart LR
    App[Node/Spring Container] --> DB[(Database Container)]

Example

version: "3.8"

services:
  app:
    build: .
    ports:
      - "3000:3000"
    depends_on:
      - postgres

  postgres:
    image: postgres:16
    environment:
      POSTGRES_USER: postgres
      POSTGRES_PASSWORD: postgres
      POSTGRES_DB: sampledb

Why Compose?

Without Compose	With Compose
Manual networking	Automatic networking
Multiple docker run commands	Single command
Hard to maintain	YAML-based configuration

Run:

docker compose up --build

Stop:

docker compose down

Why the Database Uses an Official Image

Node.js/Spring Boot applications contain custom business logic, so they require a Dockerfile.

Databases like PostgreSQL and MySQL are standardized products maintained by their vendors, so it is best practice to use their official images.

Sharing Images

Docker Hub

docker login
docker tag node-app username/node-app:1.0
docker push username/node-app:1.0

Offline

docker save -o node-app.tar node-app
docker load -i node-app.tar

Spring Boot Docker Notes

• Maven is required only to build the JAR.
• Maven is not needed in the runtime image.
• Multi-stage Docker builds are recommended for production.

Key Takeaways

• Dockerfile builds an image.
• Images become containers when executed.
• Use Docker Compose for multi-container applications.
• Use official database images whenever possible.
• Package only your application into custom images.