CI: Should I run tests directly or through Docker container?

I'm working on a new Python Flask app and am about to setup CircleCI to run our automated test suite.

We have a Dockerfile we use to deploy our app to ECS.

My question is: Is there any value in setting up CircleCI to build and run our test suite from the image defined by our Dockerfile or should we let CircleCI detect the app, setup the environment and just run the test suite directly?