azure devops pipeline issue docker not using correct python version?

I'm new to docker and microsoft azure. I've developed a python flask web app and am trying to deploy it to the azure service. Part of the deployment is passing the updated code through a pipeline. One part of the pipeline is building a docker image. Prior to this step I have a task saying to use Python 3.7. However when it builds and runs the requirments.txt file, I believe it is still using Python 2.7. How do I go about telling the docker build task the correct python version, or activating the environment so that it actually knows the correct python?