Perform a pre-flight check for basic things before deploying

Before deploying, let's provide a pre-flight check to make sure basic deployment issues are caught ahead of time.

For example:
- check for correct PORT number, e.g. if using a Dockerfile that has port 8051 defined, did the agent specify PORT=8051 to cloud run?
- check for a .gcloudignore file if a .venv or venv folder exists
- etc.