fix(docker): bump nginx to 1.31.0 (CVE-2026-42945) (#15007)

## Summary
- Bump pinned nginx in `Dockerfile` from `1.29.5-1~noble` (vulnerable)
to `1.31.0-1~noble` to remediate **CVE-2026-42945**.

## Root Cause
`Dockerfile:58` pinned `ARG NGINX_VERSION=1.29.5-1~noble`. Per the
official nginx security advisory, **CVE-2026-42945** is a buffer
overflow in `ngx_http_rewrite_module` triggered via the `rewrite` and
`set` directives, affecting nginx **0.6.27 through 1.30.0**. `1.29.5`
falls inside that range, so the shipped image is vulnerable.

References:
- nginx security advisories:
https://nginx.org/en/security_advisories.html
- Vendor advisory: https://my.f5.com/manage/s/article/K000161019
- Fixed versions: `1.31.0` (mainline) and `1.30.1` (stable)

## Fix
Single-line change in `Dockerfile:58`:

```diff
-ARG NGINX_VERSION=1.29.5-1~noble
+ARG NGINX_VERSION=1.31.0-1~noble

Dockerfile

@@ -55,7 +55,7 @@ RUN mkdir -p /usr/share/infinity/resource && \
     cp -r /tmp/resource/* /usr/share/infinity/resource && \
     rm -rf /tmp/resource
 
-ARG NGINX_VERSION=1.29.5-1~noble
+ARG NGINX_VERSION=1.31.0-1~noble
 RUN --mount=type=cache,id=ragflow_apt,target=/var/cache/apt,sharing=locked \
     mkdir -p /etc/apt/keyrings && \
     curl --retry 5 --retry-delay 2 --retry-all-errors -fsSL https://nginx.org/keys/nginx_signing.key | gpg --dearmor -o /etc/apt/keyrings/nginx-archive-keyring.gpg && \