Step 3: Building a Hello World Python Image

Objective: Write a Dockerfile to serve a python script through HTTP.


We will use port 20853 for this project.

  • You will need to create a reverse proxy to our hello world app.

  • For example, our site is

5.3.1. A Basic Python Image

  1. Let’s start by setting up our environment, which includes creating the Dockerfile and Python project files.

    • These commands create the directory and empty files required for the project


    The name of our project.


    Contains the commands used to build our image.
    • The entry point into the application.

    • It contains main

    echo "flask" > requirements.txt
    • Writes “flask” to file “requirements.txt”.

    • This project uses the Flask microframework.

    • A text file that contain the Python packages required for the project.

    • python pip will install all of these required packages.

    • It more convenient to list them in an external file instead of adding them to the Dockerfile directly.

    mkdir ~/hello-world-python
    cd ~/hello-world-python
    touch Dockerfile
    echo "flask" > requirements.txt
  2. Let’s create our Python hello-world script

    File contents of
    # hello-world-python/
    from flask import Flask
    app = Flask(__name__)
    def hello_world():
        hello = "Hello World! Привет, мир! Сәлем Әлем!\n"
        return hello
    if __name__ == "__main__":"", port=int("5000"), debug=True)
  3. Next, we’ll create the Dockerfile that we’ll use to build the image.

    You can understand most of the commands. If not, go back to the previous labs to review.

    FROM python:alpine3.11

    Specifies the base OS for our image.

    This image is alredy configured with Python and contains the required development and runtime package. There are other Python images that you can use from Docker Hub.

    COPY . /app

    Copies the contents of our project directory to the /app folder in our image.

    WORKDIR /app
    • Sets the working directory of the image.

    • This is the default directory of the user when they enter a container using a shell.

    EXPOSE 5000

    Allows outside connections to the container on this port.

    CMD python ./

    Executes using python

    File contents of Dockerfile
    # Builds a 'hello world' Python image
    FROM python:alpine3.9
    RUN apk update && \
        apk add nano curl wget
    COPY . /app
    WORKDIR /app
    RUN pip install -r requirements.txt
    EXPOSE 5000
    CMD python ./
  4. Now, we’ll build an image from the Dockerfile

    • Name the image hello-world with tag python

    docker build -t hello-world:python .
    root@vps298933:~/hello-world-python$ docker build -t hello-world:python .
    Sending build context to Docker daemon  4.096kB
    Step 1/7 : FROM python:alpine3.11
    alpine3.11: Pulling from library/python
    c9b1b535fdd9: Pull complete
    2cc5ad85d9ab: Pull complete
    61614c1a5710: Pull complete
    0522d30cde10: Pull complete
    938854eeb444: Pull complete
    Digest: sha256:50c60fffe5451e18af2c53d75b6864b5a0fcb458e239302cc218064ce4946ce7
    Status: Downloaded newer image for python:alpine3.11
     ---> a1cd5654cf3c
    Step 2/7 : RUN apk update &&     apk add nano curl wget
     ---> Running in dcfeb5a46533
    v3.11.3-75-gbcab687d4f []
    v3.11.3-79-gcdba3c9b8f []
    OK: 11268 distinct packages available
    (1/6) Installing nghttp2-libs (1.40.0-r0)
    (2/6) Installing libcurl (7.67.0-r0)
    (3/6) Installing curl (7.67.0-r0)
    (4/6) Installing libmagic (5.37-r1)
    (5/6) Installing nano (4.6-r0)
    (6/6) Installing wget (1.20.3-r0)
    Executing busybox-1.31.1-r9.trigger
    OK: 25 MiB in 41 packages
    Removing intermediate container dcfeb5a46533
     ---> 40c9154986cc
    Step 3/7 : COPY . /app
     ---> 808372ed1035
    Step 4/7 : WORKDIR /app
     ---> Running in ed1292f0232d
    Removing intermediate container ed1292f0232d
     ---> 97650e8d17c9
    Step 5/7 : RUN pip install -r requirements.txt
     ---> Running in 9cd8ea6ff985
    Collecting flask
      Downloading Flask-1.1.1-py2.py3-none-any.whl (94 kB)
    Collecting click>=5.1
      Downloading Click-7.0-py2.py3-none-any.whl (81 kB)
    Collecting itsdangerous>=0.24
      Downloading itsdangerous-1.1.0-py2.py3-none-any.whl (16 kB)
    Collecting Jinja2>=2.10.1
      Downloading Jinja2-2.11.1-py2.py3-none-any.whl (126 kB)
    Collecting Werkzeug>=0.15
      Downloading Werkzeug-1.0.0-py2.py3-none-any.whl (298 kB)
    Collecting MarkupSafe>=0.23
      Downloading MarkupSafe-1.1.1.tar.gz (19 kB)
    Building wheels for collected packages: MarkupSafe
      Building wheel for MarkupSafe ( started
      Building wheel for MarkupSafe ( finished with status 'done'
      Created wheel for MarkupSafe: filename=MarkupSafe-1.1.1-py3-none-any.whl size=12629 sha256=23579f4ba4a47cba6cace39a50c4219965aabe87d09c68fb9a2a0e48829db524
      Stored in directory: /root/.cache/pip/wheels/0c/61/d6/4db4f4c28254856e82305fdb1f752ed7f8482e54c384d8cb0e
    Successfully built MarkupSafe
    Installing collected packages: click, itsdangerous, MarkupSafe, Jinja2, Werkzeug, flask
    Successfully installed Jinja2-2.11.1 MarkupSafe-1.1.1 Werkzeug-1.0.0 click-7.0 flask-1.1.1 itsdangerous-1.1.0
    Removing intermediate container 9cd8ea6ff985
     ---> 5d090d22154a
    Step 6/7 : EXPOSE 5000
     ---> Running in d0cbf2c857ae
    Removing intermediate container d0cbf2c857ae
     ---> 7d22dfe8c2bc
    Step 7/7 : CMD python ./
     ---> Running in f87e2caaadc2
    Removing intermediate container f87e2caaadc2
     ---> c1cbde34bf35
    Successfully built c1cbde34bf35
    Successfully tagged hello-world:python
    root@vps298933:~/hello-world-python# docker images
    REPOSITORY          TAG                 IMAGE ID            CREATED              SIZE
    hello-world         python              c1cbde34bf35        About a minute ago   129MB
    python              alpine3.11          a1cd5654cf3c        2 weeks ago          109MB
    alpine-demo         test                5b250f17b87a        5 hours ago          16.8MB
    alpine              latest              5cb3aa00f899        5 weeks ago          5.53MB
  5. Create your Reverse Proxy to port 20853 before continuing.

  6. Create a temporary container of our Python app and then verify that it started correctly.


    We won’t use the daemon (background) process just yet. We’ll watch how Docker process live requests.

    Let’s review the flags args that we will use for docker run


    Automatically removes the container once it stops

    name hello-world-python

    Names our container hello-world-python

    -p 20853:5000
    • Specifies the port pairs, which are the external (20853) and internal ports (5000).

      • Port 5000 is the port that the Python process uses inside of the container.

      • Port 20853 is the port that the Docker is listening on.


    Specifies the image that the container will use

    docker run --rm --name hello-world-python -p 20853:5000 hello-world:python


    The process starts and then seems to hang, but it is still running. You are viewing the live docker container. The docker container displays the output on the screen instead of writing it to a log file. The application will exit when you press Ctrl C.

    root@vps298933:~/hello-world-python# docker run --name hello-world-python --rm -p 20853:5000 hello-world:python
     * Serving Flask app "index" (lazy loading)
     * Environment: production
       WARNING: Do not use the development server in a production environment.
       Use a production WSGI server instead.
     * Debug mode: on
     * Running on (Press CTRL+C to quit)
     * Restarting with stat
     * Debugger is active!
     * Debugger PIN: 605-515-736
    • Open up a web browser and view your page. Watch the activity from the docker container. Notice the two lines that appeared.

      1. HTTP/1.0" 200 is the response for root /

      2. HTTP/1.0" 404 is a resource not found error because the favico.ico file does not exist.

      3. Try it again. You’ll see another HTTP/1.0" 200 response.

      4. Type in an invalid file name to generate a 404 error.


    Open up another SSH instance and view the running containers if you are unsure if your reverse proxy is working correctly. Then you can test the hello world app using curl localhost:20853.

    Press Ctrl C to exit the container and return to your prompt. The container will remove itself because we started it with flag --rm.

    Output - - [16/Feb/2019 15:12:08] "GET / HTTP/1.0" 200 - - - [16/Feb/2019 15:12:10] "GET /favicon.ico HTTP/1.0" 404 - - - [16/Feb/2019 15:14:59] "GET / HTTP/1.0" 200 - - - [16/Feb/2019 15:16:34] "GET / HTTP/1.0" 404 -
  7. Start the container in daemon mode with flag -d.

    We saw that our hello world application ran in the foreground. But, we want our project to run in the background.

    Start the container with flag d and verify that the container is serving HTTP requests using the browser window or curl.

    docker run  -d --rm --name hello-world-python -p 20853:5000 hello-world:python
    root@vps298933:~/hello-world-python# docker run --name hello-world-python -d -p 20853:5000 hello-world:python
    root@vps298933:~/hello-world-python# docker ps
    CONTAINER ID        IMAGE                 COMMAND                  CREATED             STATUS              PORTS                      NAMES
    122104b14f70        hello-world:python    "/bin/sh -c 'python …"   4 seconds ago       Up 2 seconds>5000/tcp    hello-world-python
    1cd0b2b19e16        alpine-demo:daemon    "/ date"    2 hours ago         Up 2 hours                                     alpine-demod
    e0abd9624ce5        alpine-demo:test      "/bin/sh -c 'while t…"   5 hours ago         Up 5 hours                                     alpine-demo
    root@vps298933:~/hello-world-python# curl localhost:20853
    Hello World! Привет, мир! Сәлем Әлем!

5.3.2. Modify the Container Code

The image contains a basic Python script that displays some text. We don’t want to rebuild our image each time we make a change to the code.

We can make temporary changes to the running container by entering it through a shell or by modifying the files in the current directory.

  1. First, let’s enter through a terminal so that we can edit the inside of the container.

    docker exec -it hello-world-python /bin/sh

    Notice that the path in our terminal is /app because we specified that value as our working directory. Also, we copied all files from the project directory. We can see those files and modify the copies running in the container. We haven’t specified any volumes. So, these changes are confined to this container.

    root@vps298933:~/hello-world-python# docker exec -it hello-world-python /bin/sh
    /app # ls -lh
    total 12
    -rw-r--r--    1 root     root         203 Feb 17 04:10 Dockerfile
    -rw-r--r--    1 root     root         238 Feb 16 15:27
    -rw-r--r--    1 root     root           6 Feb 15 16:11 requirements.txt
    /app #
  2. Now, we can edit the file using nano because we included that package in our image.

    1. Modify the Hello World text.

      For example, add more languages to our app! Notice the \n characters at the end of the text. That character specifies a newline, such as pressing the Enter key.

      hello = "Hello World! Привет, мир! Сәлем Әлем! ¡Hola Mundo! नमस्ते दुनिया! 안녕하세요! Hallo Welt! 你好,世界!\n"
      return hello
  3. Save the file and test the new code using your web browser.

    Or, exit the terminal using the exit command and then test using curl.

     root@vps298933:~/hello-world-python# docker exec -it python /bin/sh
     /app # nano
     /app #
     /app # exit
     root@vps298933:~/hello-world-python# curl localhost:20853
     Hello World! Привет, мир! Сәлем Әлем! ¡Hola Mundo! नमस्ते दुनिया! 안녕하세요! Hallo Welt! 你好,世界!

5.3.3. Mounting a Volume

Modifying the code inside of the container does not save the data without a mount point. You should create a volume so that you can update or save your app data without rebuilding an image.

  1. Stop and remove the container.

  2. Start the container with a mounted volume

    1. Restart it with a volume mounted to the current directory.

    2. Set the restart flag so that the app will automatically restart

      • Volume: -v ~/hello-world-python:/app

      • Restart flag: --restart always

    docker run -d -v ~/hello-world-python:/app --restart always --name hello-world-python -p 20853:5000 hello-world:python


Now, you can modify or other code files in your app directory from the host machine or container without losing the data when the container is removed.

nano ~/hello-world-python/

5.3.4. Wrap-up

We learned how to use Docker to create and then deploy a simple Hello World Python application.

You know how to

  1. write a Dockerfile for a custom project.

  2. use docker run to create a container.

  3. create a docker-compose.yml file.

  4. create volumes to separate that dynamic data from the process.

You now have the knowledge and skill required to package your Python app in a Docker image for quick deployment.

This lab is just the beginning. Python was the example that we used here. You can repeat the process using other languages.