This is an example of a Python-based Docker Dev Environment that shows how to set environment variables with regards this issue.
Open Docker Desktop and create a Dev Environment from https://github.com/felipecruz91/python-dev-env
Clikc on Open in VSCode and run the python script within the dev container:
python -u hello_world.py
The script will print the value of the environment variable RUNNING_IN_CONTAINER.
The output should be 1 as it is the value set in the Dockerfile.devenv