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Docker containers bundle everything an application needs to run—and nothing more—making it simple to move workloads across environments. In this guide, you’ll learn how to:
  1. Define your app environment with a Dockerfile
  2. Build a container image
  3. Push the image to a registry (e.g., Docker Hub)
  4. Run the image locally

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1. Writing a Dockerfile

Start by creating a Dockerfile in your project root. This example shows a Spring Boot application running on OpenJDK 8 (Alpine): 
# Use OpenJDK 8 on Alpine Linux as the base image
FROM openjdk:8-jdk-alpine

# Document the port the application listens on
EXPOSE 9999

# Build-time variable for the JAR file
ARG JAR_FILE=target/*.jar

# Copy the packaged JAR into the container
ADD ${JAR_FILE} app.jar

# Run the JAR file when the container starts
ENTRYPOINT ["java", "-jar", "/app.jar"]
Key directives:
  • FROM sets the base image.
  • EXPOSE documents which port the app listens on.
  • ARG defines a build-time variable for your JAR.
  • ADD copies your artifact into the image.
  • ENTRYPOINT specifies the startup command.

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2. Building the Docker Image

Run this command from the directory containing your Dockerfile. Replace <docker-hub-username> and <image-name> as needed: 
docker build -t <docker-hub-username>/<image-name>:v1 .
  • -t tags your image in the format repository/name:tag.
Docker caches each layer by default. When you rebuild without changing earlier steps, subsequent builds are faster. To bypass the cache, add --no-cache.

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3. Pushing to Docker Hub

First, authenticate using: 
docker login
Then push your tagged image: 
docker push <docker-hub-username>/<image-name>:v1
You must be logged in to Docker Hub before pushing images.
Use docker login and enter your credentials when prompted.

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4. Running the Container Locally

Map port 9999 on your host to port 9999 in the container: 
docker run -d -p 9999:9999 <docker-hub-username>/<image-name>:v1
  • -d runs the container in detached mode.
  • -p host_port:container_port publishes container ports to the host.

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Quick Reference: Docker CLI Commands

CommandPurposeExample
docker buildBuild an image from a Dockerfiledocker build -t user/app:latest .
docker pushPush an image to a registrydocker push user/app:latest
docker loginAuthenticate with a Docker registrydocker login
docker runCreate and start a container from an imagedocker run -d -p 8080:8080 user/app:latest
docker imagesList all local imagesdocker images
docker psList running containersdocker ps

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