Besides main.py, you have to have two files in the local folder:

requirements.txt

Where you define the libraries and their version, according to pypi.org

requests==2.31.0

Dockerfile

FROM python:3.9

EXPOSE 8080
ENV PORT 8080

WORKDIR /home

COPY . /home
RUN pip install -r /home/requirements.txt

CMD python3 /home/main.py

Your Flask application should be something like below:

import os

from flask import Flask

app = Flask(__name__)


@app.route("/")
def hello_world():     
    return "Hello World!"


if __name__ == "__main__":
    app.run(debug=True, host="0.0.0.0", port=int(os.environ.get("PORT", 8080)))

See this official documentation for step by step instruction: Deploy a Python service to Cloud Run

There is a plugin called: Cloud Code IDE plugin which makes the test and deployment easy. I am using it for VS code, once the initial setups and permissions are taken care, few clicks, you will be able to run locally, debug and deploy Cloud run services from your local instance.