> ## Documentation Index
> Fetch the complete documentation index at: https://notes.kodekloud.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Docker Basics

> Learn to containerize applications using Docker by defining environments, building images, pushing to registries, and running containers locally.

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

***

## 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):

```dockerfile theme={null}
# 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.

***

## 2. Building the Docker Image

Run this command from the directory containing your `Dockerfile`. Replace `<docker-hub-username>` and `<image-name>` as needed:

```bash theme={null}
docker build -t <docker-hub-username>/<image-name>:v1 .
```

* `-t` tags your image in the format `repository/name:tag`.

<Callout icon="lightbulb" color="#1CB2FE">
  Docker caches each layer by default. When you rebuild without changing earlier steps, subsequent builds are faster. To bypass the cache, add `--no-cache`.
</Callout>

***

## 3. Pushing to Docker Hub

First, authenticate using:

```bash theme={null}
docker login
```

Then push your tagged image:

```bash theme={null}
docker push <docker-hub-username>/<image-name>:v1
```

<Callout icon="triangle-alert" color="#FF6B6B">
  You must be logged in to [Docker Hub](https://hub.docker.com/) before pushing images.\
  Use `docker login` and enter your credentials when prompted.
</Callout>

***

## 4. Running the Container Locally

Map port 9999 on your host to port 9999 in the container:

```bash theme={null}
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.

***

## Quick Reference: Docker CLI Commands

| Command       | Purpose                                    | Example                                      |
| ------------- | ------------------------------------------ | -------------------------------------------- |
| docker build  | Build an image from a Dockerfile           | `docker build -t user/app:latest .`          |
| docker push   | Push an image to a registry                | `docker push user/app:latest`                |
| docker login  | Authenticate with a Docker registry        | `docker login`                               |
| docker run    | Create and start a container from an image | `docker run -d -p 8080:8080 user/app:latest` |
| docker images | List all local images                      | `docker images`                              |
| docker ps     | List running containers                    | `docker ps`                                  |

***

## Links and References

* [Docker Documentation](https://docs.docker.com/)
* [Dockerfile reference](https://docs.docker.com/engine/reference/builder/)
* [Docker Hub | Official Images](https://hub.docker.com/search?q=\&type=image)

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