After reading the previous blog post in this series, "Containers, Kubernetes, microservices: Start here", you're now ready to build your first "Hello World" application and run it in a container. For this, we'll be using Python.
Buildah, Podman, or docker
Which method you use to build and run your container is based on your operating system and tool selection. Because it is safer (it does not require root access), I'm going to use Podman to build and run my container, knowing that the commands used are 100 percent compatible with the docker command. In fact, you could run alias docker=podman and you'd not know the difference. So, if you are not using podman, simply use the command docker in place of every podman in the following.
Parts For Building
You need the code you're going to run, a file to configure/manage the build process, and the tool (i.e. Podman).
Build Configuration/Management
We'll create a file called "Dockerfile" that contains the steps and information needed to build an image. The build process is done in layers, with the starting point typically being an operating system or, more likely, an OS and framework combination. For example, you need Python 3 installed. So it's pretty common to start from that point. You can -- and there might be good reasons for this -- start with the operating system and then install the framework all inside your image as you build it. You can also do just that -- start with an OS and add a framework -- and save that image and use it as your base for other images. Yes, like any IT technology, you can make this as simple or as complicated as you like. We'll start with an OS+Framework combination to keep things simple. We'll then copy our code in the image, then make sure the dependencies are installed. Then, we'll give the image a command to be executed when someone runs the image in a container. The following file, "Dockerfile" does those things:
FROM python:3ADD helloworld.py /RUN pip install flaskRUN pip install flask_restfulEXPOSE 3333CMD [ "python", "./helloworld.py"]Let's Get Some Code
Let's Build And Run
podman build -t hello-world-python .podman run -p 3333:3333 hello-world-pythoncurl http://localhost:3333The Cycle
- Create the source code
- Create a Dockerfile file
- Build the image
- Run the image in a container
About that Dockerfile
The file “Dockerfile” is used to guide the construction of your image. Here’s a short, step-by-step breakdown: FROM python:3 This is your base image, the starting point. In this case, it’s the official image from the Python Software Foundation and has Python:3 installed. That means we don’t have to install any framework; it’s already included with this base image. In case you’re wondering, it’s 929MB on my Mac. 
ADD helloworld.py /RUN pip install flaskRUN pip install flask_restfulEXPOSE 3333CMD [ "python", "./helloworld.py"]podman run or docker run).Running In A Container
Running the podman run -p 3333:3333 hello-world-pythoncommand starts the image in a container. It code uses port 3333, and it is mapped to the local port 3333. Feel free to experiment with this. It will be attached to your command line; that is, it ties up your terminal while it’s running. You can eliminate this by using the --detach option in your command. In that case, the container runs in the background.
You can see the results of the code by running the curl command or opening your browser to http://localhost:3333.
Containerize All The Things
So you know have all the knowledge and tool necessary to run your Node.js code in a Linux container. Expanding this knowledge to include multiple instances of an application, and/or multiple applications in a cluster, is for the next blog post.
