Docker app cannot be accessed through opened ports
So based on the comments it's clear you're not actually running on 0.0.0.0 and that's probably (noticing your command for the container) because you're running a newer version of flask where I think you have to pass some args to flask run.
Try flask run --host=0.0.0.0 as the command in your container and I think that will probably work out how you're expecting :)
More info at the Flask Docs.