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4.1. Understanding Dockerfile Commands

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This is an optional chapter designed to help you deeply understand what each line in the Dockerfile accomplishes.

Docker Architecture Overview​

architecture-beta
    group container(cloud)[Docker Container]
    
    service jre(server)[Java 17 JRE] in container
    service app(disk)[/app directory] in container
    service jar(database)[app.jar] in container
    
    jre:R --> L:jar

1. Base Image​

FROM eclipse-temurin:17-jre

This tells Docker to start with a lightweight Alpine Linux distribution that comes pre-installed with the Java 17 Runtime Environment.

2. Working Directory​

WORKDIR /app

This creates a folder named /app inside the Docker image and sets it as the default directory for all subsequent commands.

3. Copying the Artifact​

COPY target/bigkart-0.0.1-SNAPSHOT.jar app.jar

This takes the JAR file from your local target folder and copies it into the /app folder inside the image, renaming it to simply app.jar.

4. Exposing Ports​

EXPOSE 8080

Since the Docker Image is essentially a virtual environment, we must explicitly expose port 8080 so that external traffic can reach our Spring Boot application.

5. Execution Command​

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

This defines the default command that runs when the container starts up. It executes the Spring Boot application!