Get Started with Docker. Create a Docker Image

Get Started with Docker. Create a Docker Image

This tutorial contains step-by-step instructions on creating your first Docker image and container for a basic web server app written in Python.

After creating a Docker container with your application, you can deploy it to your Kubernetes cluster.

This tutorial is for Linux users.

Docker Image Definition

A Docker image provides an isolated filesystem to run the container. The image must contain everything needed to run your application — the operating system, all dependencies, configurations, scripts, binaries, etc. The image also contains configuration data for the container, such as environment variables, a default command to run, and other metadata.

Docker Container Definition

A Docker container is a sandboxed process running on a host machine. A container:

Is a runnable instance of an image.
Can be run on local machines, virtual machines, or deployed to the cloud.
Is portable and can be run on any OS.
Is isolated from other containers and runs its own software, binaries, configurations, etc.

Prerequisites

You have installed the latest version of Docker Desktop or Docker Engine.
You have a text editor to edit files.

Create the Directory Structure

Create the directories to store your application source code and Docker metadata:

# Your project root
mkdir quickstart_docker
# Application source code
mkdir quickstart_docker/application
# Docker configuration files
mkdir quickstart_docker/docker
# Dockerfile location
mkdir quickstart_docker/docker/application

[!NOTE]
This is a suggested directory structure to organize the Docker and application files. You can use a directory structure better suited for your project.

Go to the quickstart_docker directory. Replace /path/to/quickstart_docker with the path to your quickstart_docker directory:
```
cd /path/to/quickstart_docker
```
Create the application file:
```
touch application/application.py
```

Using a text or code editor, add the sample HTTP server Python code in the application.py file:

import http.server
import socketserver
import os

# Listen to port 8800
PORT = 8800

# Serve files from the current directory
# directly mapping the directory structure to HTTP requests
Handler = http.server.SimpleHTTPRequestHandler

# Create and listen at the HTTP socket, 
# dispatching the requests to the handler.
httpd = socketserver.TCPServer(("", PORT), Handler)

# Echo current status to the console and server logs. 
# It will be shown as the first line in the 'docker logs' command output.
os.system(f"echo 'serving at port {PORT}'")

# Handle requests until the server is terminated.
httpd.serve_forever()

Create the Dockerfile:
```
touch docker/application/Dockerfile
```

Add the following Docker image configuration in the Dockerfile:

# Use the base Python image from the Docker Hub
FROM python:3.9

# Set the working directory of the Docker container to '/app'. 
# Any RUN, CMD, ADD, COPY, and ENTRYPOINT 
# commands will be executed in the specified directory.
WORKDIR /app

# Copy the 'application' directory contents into the container at '/app'
COPY ./application /app

# Make port 8800 available to the world outside the container
EXPOSE 8800

# Execute 'python3 /app/application.py' when the container launches
CMD ["python3", "/app/application.py"]

View the contents of the quickstart_docker directory, using the tree command. You should see the following structure:

└── quickstart_docker
    ├── application
    │   └── application.py
    └── docker
        └── application
            └── Dockerfile

Build the Application Image

Build your application image using the docker build command:
```
docker build . -f docker/Dockerfile -t exampleapp
```
The docker build command above uses the following options:
- . — use the current directory as the build context.
- -f — specify the Dockerfile path: docker/Dockerfile.
- -t — tag the image, giving it a human-readable name: exampleapp. You can refer to the image name when running the container.
View the image list:
```
docker images
```

You should see the following:

REPOSITORY   TAG       IMAGE ID       CREATED       SIZE
exampleapp   latest    22a44e06fe8d   1 minute ago  997MB

Create and Test the Container

Create and run a container from your image:
```
docker run --name server -p 128.0.0.1:8800:8800 -d exampleapp
```
The docker run command above uses the following options:
- -p — publish the container's port 8800 to 127.0.0.1:8800 (localhost:8800) on the host. Without this port mapping, you won't be able to access the application from the host.
- -d — run the container in the background.
- --name — assign the name server to the container.
- exampleapp — the name of the image to containerize.

You should see the container ID, similar to the following:

f7a6449f80ee87003e40ef7746fd102695c4910ee94906e0711f97ca1765068f

Open your web browser to http://localhost:8800. You should see the contents of the /app directory, served by the application in the container:
Stop the container:
```
docker container stop server
```