Dockerfile Basics

Introduction

In the previous section, we learned about what Docker is and how it can be used to containerize applications. Now, let's dive into creating a simple Dockerfile to build an image. A Dockerfile is a text document that contains all the commands a user could call on the command line to assemble an image. Using docker build, users can create an automated build that executes several command-line instructions in succession.

Concept

A Dockerfile consists of a series of instructions, each one creating a new layer in the image. The basic syntax for a Dockerfile is as follows:

• FROM: Specifies the base image to use.
• RUN: Executes any commands in a new layer on top of the current image and commits the results.
• CMD: Provides defaults for executing containers.
• EXPOSE: Informs Docker that the container listens on the specified network ports at runtime.
• WORKDIR: Sets the working directory inside the container.

Examples

Let's create a simple Dockerfile to build an image that runs a basic "Hello, World!" application using Python.

Step 1: Create a Directory for Your Project

First, create a new directory for your project and navigate into it:

mkdir hello-world-docker
cd hello-world-docker

Step 2: Create a Python Script

Create a simple Python script that prints "Hello, World!".

1# app.py
2print("Hello, World!")

Step 3: Create a Dockerfile

Next, create a Dockerfile in the same directory. This file will contain instructions to build the Docker image.

1# Dockerfile
2FROM python:3.9-slim
3 
4WORKDIR /app
5 
6COPY app.py .
7 
8CMD ["python", "app.py"]

• FROM python:3.9-slim: This line specifies that we are using the official Python 3.9 slim image as our base.
• WORKDIR /app: Sets the working directory inside the container to /app.
• COPY app.py .: Copies the app.py file from your host machine into the /app directory in the container.
• CMD ["python", "app.py"]: Specifies the command to run when a container is started, which in this case is running the Python script.

Step 4: Build the Docker Image

Now that you have your Dockerfile and application ready, you can build the Docker image. Run the following command:

You should see the output:

Hello, World!

What's Next?

In this tutorial, we learned how to create a simple Dockerfile and build an image. In the next section, we will explore Multi-Stage Builds, which allow you to optimize your images by reducing their size and improving security.

Stay tuned for more advanced topics in our Docker curriculum!