Deploying a minimal flask app in docker - server connection issues

Question

I have an app who\'s only dependency is flask, which runs fine outside docker and binds to the default port 5000. Here is the full source:

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Answer 1

To build on other answers:

Imagine you have two computers. Each computer has a network interface (WiFi, say), which is its public IP. Each computer has a loopback/localhost interface, at 127.0.0.1. This means "just this computer."

If you listed on 127.0.0.1 on computer A, you would not expect to be able to connect to that via 127.0.0.1 when running on computer B. After all, you asked to listen on computer A's local, private address.

Docker is similar setup; technically it's the same computer, but the Linux kernel is allowing each container to run with its own isolated network stack. So 127.0.0.1 in a container is the same as 127.0.0.1 on a different computer than your host—you can't connect to it.

Longer version, with diagrams: https://pythonspeed.com/articles/docker-connection-refused/