fix(docker): fix startup crash and permission errors in the container

Four related fixes uncovered while getting Docker running on a Linux
host without AVX-512 and with an NVIDIA GPU:

- pedalboard>=0.9.21 ships a Linux wheel with AVX-512 instructions
  baked into its native extension, which SIGILLs (exit 132) on import
  on any CPU without AVX-512 support. Pin below the regression until
  upstream fixes it (spotify/pedalboard#454).
- The rocminfo probe in app.py ran unconditionally on every build
  variant, always failing and logging on non-ROCm systems. Gate it on
  /dev/kfd actually being present.
- sox (required by qwen-tts's X-vector extractor) and a C compiler
  (required by Triton to JIT-compile its CUDA driver shim on first
  use) were missing from the runtime image, causing hard failures
  once those code paths were actually exercised.
- Named volumes and bind mounts (HF cache, app data, generated audio)
  are created root-owned by Docker on first use, but the app runs as
  the unprivileged voicebox user. chown the mount points in the
  entrypoint before dropping privileges so this self-heals on every
  start regardless of host UID.

Dockerfile

@@ -84,12 +84,20 @@ RUN mkdir -p /app/data/generations \
 
 WORKDIR /app
 
-# Install only runtime system dependencies (gosu drops root in the entrypoint)
+# Install only runtime system dependencies (gosu drops root in the entrypoint).
+# sox is required by qwen-tts's speech_vq X-vector extractor, which shells
+# out to it via the `sox` Python bindings for reference-audio normalization.
+# gcc/g++ are required by Triton (used by some torch ops, e.g.
+# bmm_outer_product) to JIT-compile its CUDA driver shim on first use --
+# without them, PyTorch's CUDA-capable build crashes with "Failed to find
+# C compiler" even when a GPU is actually present.
 RUN apt-get update && apt-get install -y --no-install-recommends \
     ffmpeg \
+    sox \
+    gcc \
+    g++ \
     curl \
     gosu \
-    sox \
     && rm -rf /var/lib/apt/lists/*
 
 # Copy installed Python packages from builder stage