relay fails to register with web - web resets the connection

### Self-Hosted Version

25.10.0

### CPU Architecture

x86_64

### Docker Version

29.1.3

### Docker Compose Version

5.0.0

### Machine Specification

- [x] My system meets the minimum system requirements of Sentry

### Installation Type

Was fresh install some time ago (23.something?), then upgraded to 25.10

### Steps to Reproduce

1. Run `./install.sh`
2. Run `docker compose up -d`
3. Run `docker compose logs -n 100 -f relay`

### Expected Result

Sentry to be up and running, processing incoming reports.

### Actual Result

Sentry starts and the Web UI is fully usable.

relay repeatedly fails to register with the error
```
relay-1 | 2025-12-15T19:51:26.067553Z ERROR relay_server::services::upstream: authentication encountered error error=could not send request to upstream error.sources=[error sending request for url (http://web:9000/api/0/relays/register/response/), client error (SendRequest), connection closed before message completed] tags.attempts=20
```

This looks exactly like https://github.com/getsentry/relay/issues/1096

While I don't want to upload the pcap I took in case it contains secrets, the web node resets the connection to relay 3ms after it sends the register/response HTTP call as seen in the below screenshot of the pcap:

It seems this isn't completely uncommon by the issue raised on Relay several years ago that was never fixed.

The HTTP call to `/api/0/relays/register/response/` is never logged by the web container at all.

### Event ID

_No response_

Reproduce with install.sh and docker compose up -d, then inspect the relay logs around /api/0/relays/register/response/. Compare the connection reset with relay issue 1096 and verify whether the web container receives the request. Done means relay registers successfully and Sentry processes incoming reports.

Written by the indexing model from the issue text.