Introduction

Docker revolutionizes software development by enabling developers to package applications into containers—standardized executable components combining application source code with the operating system libraries and dependencies required to run that code in any environment. One of Docker’s key advantages lies in its predictive build process, which ensures consistency across all stages of development.

This guide will walk you through the process of building Docker images from scratch, an essential skill for modern developers working within cloud-based ecosystems.

Prerequisites

• A basic understanding of Docker’s purpose and functionality. If you need to get started, consider checking out our guide to managing Docker networks here.
• Docker installed on your machine. Visit the official Docker site (Official site) for installation instructions.
• A working text editor such as Visual Studio Code or Sublime Text.

Step 1: Setting Up the Workspace

Begin by creating a new directory for your project and navigating into it:

mkdir docker-image-demo
cd docker-image-demo

Step 2: Creating the Dockerfile

The Dockerfile is a text document that contains all the commands to assemble an image. Create a new Dockerfile using your text editor:

touch Dockerfile

Open the Dockerfile and add the following example base image and command set:

FROM python:3.9-slim
COPY . /app
WORKDIR /app
RUN pip install -r requirements.txt
ENTRYPOINT [ \"python\", \"app.py\" ]

UNDERSTANDING THE COMMANDS

• FROM: Specifies the base image to build your application upon.
• COPY: Copies files from the local filesystem to the Docker container.
• WORKDIR: Sets the working directory for the application.
• RUN: Executes commands in a new layer on top of the existing image and commits the results.
• ENTRYPOINT: Configures the container to run as an executable.

Step 3: Adding Application Files

Create a simple application within the same directory. For instance, add an `app.py` file and a `requirements.txt`:

# app.py
print(\"Hello, Docker!\")

Leave `requirements.txt` empty for this basic application.

Step 4: Building the Docker Image

In your terminal, execute the following command to build your Docker image:

docker build -t my-python-app .

• -t: Option allows you to name and tag your image.
• .: Indicates to Docker to find the Dockerfile in the current directory.

Step 5: Running the Image

With the image built, run it with the following command:

docker run my-python-app

If successful, the terminal should output “Hello, Docker!”

Troubleshooting Common Issues

• Build Errors: Ensure your Dockerfile syntax is correct and paths specified are accessible.
• Run Errors: Verify your code dependencies and check Docker’s logs to pinpoint issues.

Summary Checklist

Here’s a quick checklist to ensure you remember the essential steps:

• Ensure Docker is installed and running.
• Set up your project directory and Dockerfile appropriately.
• Include necessary application files and ensure they’re accessible in your `Dockerfile`.
• Build and run your Docker image, checking for typical errors if you encounter issues.