fix(docker): bump builder from go 1.23 to 1.25 (matches go.mod)

The release.yml build on main HEAD c0185357 failed with:
  go: go.mod requires go >= 1.25.0 (running go 1.23.12; GOTOOLCHAIN=local)

go.mod was bumped to 1.25.0 during the M7/M8 batch but the
Dockerfile's builder stage still pinned golang:1.23-bookworm. Bump
to golang:1.25-bookworm to match.

Dockerfile

@@ -7,7 +7,7 @@ RUN npm ci
 COPY web/ ./
 RUN npm run build
 
-FROM golang:1.23-bookworm AS builder
+FROM golang:1.25-bookworm AS builder
 WORKDIR /src
 COPY go.mod go.sum ./
 RUN go mod download