Basic web application created using Python's Flask
To create an image of an application. Decide on a base OS, install python and and add the flask module to it, You load the app inside the container and You tell flask to run the file file pointed to on the specified host.
Flask is like a Python web framework, similar to what Express is to Node. In essence we've created a Flask application called app.py. In the Dockerfile we define the environment that the application will run on. Environment is a combination of OS and packages installed on that OS that the application will need.
Docker is a template on how to create an image.