Introduction

Goal of the Tutorial

This tutorial aims to introduce best practices for writing Dockerfiles. By following these practices, you can ensure that your Dockerfiles are efficient, readable, and maintainable.

Learning Objectives

By the end of this tutorial, you will:

• Understand the role and structure of Dockerfiles
• Know how to write efficient and maintainable Dockerfiles
• Be able to apply best practices when writing Dockerfiles

Prerequisites

To get the most out of this tutorial, you should have:

• A basic understanding of Docker
• Familiarity with command line interfaces
• Docker installed on your machine

Step-by-Step Guide

Dockerfiles are scripts that contain collections of commands you would ordinarily execute manually in order to build a Docker image. Here are the best practices you should follow when creating your Dockerfiles.

Best Practices

• Use a .dockerignore file: Just like using a .gitignore file, a .dockerignore file helps to exclude files that are not necessary for building a Docker image, thus speeding up the build process.

• Avoid installing unnecessary packages: This helps to reduce complexity, reduce dependencies, reduce the size of the image, and speed up the build by avoiding unnecessary compile time.

• Each Dockerfile should have only one responsibility: Following the 'one concern per Dockerfile' principle makes your Dockerfile more readable, maintainable, and means that each container can be independently scaled.

• Use multi-stage builds: This is a new feature in Docker that helps to drastically reduce the size of your Docker images.

• Minimize the number of layers: Each RUN, COPY, and ADD command creates a new layer in the image. Minimize the number of these commands to reduce the size of the image.

Code Examples

Example 1: A Basic Dockerfile

Here's a simple Dockerfile example:

# Use an official Python runtime as a parent image
FROM python:3.6-slim

# Set the working directory in the container to /app
WORKDIR /app

# Add the current directory contents into the container at /app
ADD . /app

# Install any needed packages specified in requirements.txt
RUN pip install --no-cache-dir -r requirements.txt

# Make port 80 available to the world outside this container
EXPOSE 80

# Run app.py when the container launches
CMD ["python", "app.py"]

This Dockerfile does the following:

• Starts from a parent image that provides Python 3.6 in a slim version
• Sets the working directory inside the container to /app
• Copies the current directory (.) from the host to /app inside the container
• Installs the Python dependencies from requirements.txt
• Exposes port 80 from the container to the host
• Specifies what command to run when the container starts

Example 2: Using .dockerignore

Create a .dockerignore file in the same directory as your Dockerfile with the following content:

*.pyc
*.pyo
*.pyd
__pycache__
*.so
*.o
*.out
.DS_Store

This will prevent all these types of files from being added to the Docker image when you use the ADD or COPY instructions.

Summary

In this tutorial, we've covered the best practices for writing Dockerfiles, such as using a .dockerignore file, minimizing the number of layers, and using multi-stage builds. With these practices, you can create efficient, readable, and maintainable Dockerfiles.

Practice Exercises

1. Write a Dockerfile for a simple Flask application.
2. Modify the Dockerfile to use multi-stage builds and compare the size of the resulting Docker image.
3. Create a .dockerignore file and include files and directories that should not be added to the Docker image.

You can refer back to the code examples and best practices discussed in this tutorial to help you complete these exercises.

Further Practice

For further practice, you can try optimizing Dockerfiles for different types of applications or explore how to use Docker Compose to manage multi-container applications.