[Bug]: Duplicate `Server` and `Date` headers from services without a gateway

[jvstme]

Steps to reproduce

1. Run the service:

   type: service
   gateway: false
   auth: false
   image: nginx
   port: 80

2. Request the service and inspect the response headers.

Actual behaviour

The Server and Date headers are duplicated. One version is coming from the origin server, while the other is added by the in-server proxy.

< HTTP/1.1 200 OK
< date: Fri, 08 May 2026 14:25:57 GMT
< server: uvicorn
< server: nginx/1.29.8
< date: Fri, 08 May 2026 14:26:01 GMT
< content-type: text/html
< content-length: 896
< last-modified: Tue, 07 Apr 2026 11:37:12 GMT
< connection: keep-alive
< etag: "69d4ec68-380"
< accept-ranges: bytes

Expected behaviour

No duplicate headers, headers from the proxy overwrite headers from the origin.

dstack version

0.20.19

Server logs

Additional information

No response