How to deal with Python's requirements.txt while using a Docker development environment?

Suppose I wrote a docker-compose.dev.yml file to set the development environment of a Flask project (web application) using Docker. In docker-compose.dev.yml I have set up two services, one for the database and one to run the Flask application in debug mode (which allows me to do hot changes without having to recreate/restart the containers). This allows everyone on the development team to use the same development environment very easily. However, there is a problem: it is evident that while developing an application it is necessary to install libraries, as well as to list them in the requirements.txt file (in the case of Python). For this I only see two alternatives using a Docker development environment:

1. Enter the console of the container where the Flask application is running and use the pip install ... and pip freeze > requirements.txt commands.
2. Manually write the dependencies to the requirements.txt file and rebuild the containers.

The first option is a bit laborious, while the second is a bit "dirty". Is there any more suitable option than the two mentioned alternatives?

Edit: I don't know if I'm asking something that doesn't make sense, but I'd appreciate if someone could give me some guidance on what I'm trying to accomplish.

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