whisper-server: Debian trixie base — Mesa 25 for the R9700 (RDNA 3.5)

bookworm's Mesa 22.3 has no gfx1151 (Strix Halo) support in RADV, so
the container silently fell back to CPU despite gpu=auto: whisper
logged 'whisper_backend_init_gpu: no GPU found'. trixie ships Mesa
25.0.7, which supports the R9700; whisper.cpp builds unchanged on it.

server/whisper-server/Dockerfile

@@ -8,7 +8,7 @@
 # Note: ggml builds with -march=native by default, so build the image ON
 # the machine that will run it (docker compose build on the server).
 
-FROM debian:12-slim AS build
+FROM debian:trixie-slim AS build
 
 RUN apt-get update && apt-get install -y --no-install-recommends \
         git cmake build-essential ca-certificates curl \
@@ -31,10 +31,11 @@ RUN cmake -S /src -B /src/build -DCMAKE_BUILD_TYPE=Release \
         -DWHISPER_VULKAN=ON \
     && cmake --build /src/build --target whisper-server -j"$(nproc)"
 
-FROM debian:12-slim
+FROM debian:trixie-slim
 
 # libvulkan1 + mesa-vulkan-drivers: the AMD RADV Vulkan driver used by
-# the R9700. GPU access is granted via /dev/dri in docker-compose.yml.
+# the R9700. trixie's Mesa (25.x) is required — bookworm's 22.x has no
+# RDNA 3.5 (gfx1151) support and whisper silently falls back to CPU.
 RUN apt-get update && apt-get install -y --no-install-recommends \
         libgomp1 ca-certificates curl libvulkan1 mesa-vulkan-drivers \
     && rm -rf /var/lib/apt/lists/*