The instructions require you to supply any published image name that includes the software you require. This could mean a custom image of your own which includes Python and also 3rd party Python libraries you will require, so you do not have to perform an install with
pip3 every time.
If you need just a vanilla image of Python 3 (with only standard libraries/built-ins included), you can consider using these: https://hub.docker.com/_/python
# This can be any image that has the necessary runtime environment for your project.
# As an example, we're using a simple Python 3 image from Docker: https://hub.docker.com/_/python
script: sleep 60