Inconsistent base image in multistage Dockerfile

### What happened?

From the Dockerfile comment:

```
# It's important that this is Debian 12 to match the distroless image.
FROM debian:13-slim AS build
...
FROM gcr.io/distroless/python3-debian12 AS image
```

It looks like renovate upgraded only the first image, leaving both the comment and the second image hanging in Debian 12.

### How can we reproduce it?
n/a

### What environment did it happen in?
Function version: main

Open the Dockerfile and compare the Debian version in the build-stage FROM line with the distroless Python image and the accompanying comment. Align the base-image versions so they consistently target Debian 12, then verify the Docker build completes successfully.

索引モデルが issue の本文から書いたものです。