Why is a docker container only connecting using 0.0.0.0? - docker

Question

I have a docker-compose containing two services:
a Python uWSGI app on port 5000 and
an Nginx reverse proxy on port 433 (with a self-signed certificate).
The latter has been set up to proxy requests into the service running in the first container, which is doing it fine.
The Python service has an endpoint that receives a POST request with a JSON payload (an object) containing a $schema attribute that allows the service to validate it:
"$schema": "https://localhost/schemas/schema.json"
The interesting part is that this schema has been set up on the service itself, which means that the attribute passed in - as the URL indicates - is being served by it. The problem happens when the service tries to validate a payload against a schema that it cannot GET (requesting the URL above).
My conclusions
I can understand why fetching from localhost fails, as there isn't any service running on localhost, 443 in the respective container:
jsonschema.exceptions.RefResolutionError: HTTPSConnectionPool(host='localhost', port=443):
Max retries exceeded with url: /schemas/schema.json
(Caused by NewConnectionError('<urllib3.connection.HTTPSConnection object at 0x7f7c26d9f5b0>:
Failed to establish a new connection: [Errno 111] Connection refused'))
As an alternative, I tried to use the IP for the Nginx container (https://nginx/schemas/schema.json), which actually finds the container, but goes through the SSL verification on the self-signed certificate.
jsonschema.exceptions.RefResolutionError: HTTPSConnectionPool(host='nginx', port=443):
Max retries exceeded with url: /schemas/schema.json
(Caused by SSLError(SSLCertVerificationError(1, '[SSL: CERTIFICATE_VERIFY_FAILED] certificate verify failed: self signed certificate (_ssl.c:1108)')))
In a third attempt, I used 0.0.0.0 which correctly routes the request and validates the object correctly.
However, I couldn't understand why it worked. As well as why the service is able to make the request through the proxy but I cannot CURL myself inside the Python service container simply using curl --insecure https://0.0.0.0/...
Any help in figuring out the real cause or a better understanding of how networking should work here would be very appreciated.