For testing applications running on Google's App Engine you need to have python configured with the google-cloud-sdk. Annoyingly these libraries are not something that can be pip installed like other dependencies, they must be installed into the OS.
This is simply a python 2.7 image with google-cloud-sdk installed and the PYTHONPATH set up so that the base modules can be imported. We need this as a docker container for CI systems such as CircleCI or bitbucket pipelines where we provide our own runtime.
Almost certainly the first thing you are going to want to do in python is:
>>> import dev_appserver
>>> dev_appserver.fix_sys_path()to fix the path to include the rest of the google specific libraries. It's so common in fact that we should probably do it in the image - but that wouldn't work very well with our existing tests so for now we just set up PATH so we can import dev_appserver.