Dockerfile: add pkg-config + libav dev headers so PyAV (faster-whisper dep) builds

Dockerfile

@@ -12,10 +12,14 @@ ENV OMP_NUM_THREADS=1 \
     TRANSFORMERS_CACHE=/tmp/huggingface/transformers \
     HUGGINGFACE_HUB_CACHE=/tmp/huggingface/hub
 
-# System libraries: ffmpeg (audio), libsndfile1 (soundfile/librosa),
-# git (some pip installs), and build tools for any source-only wheels.
+# System libraries:
+#   ffmpeg / libsndfile1   - audio decode for whisper, soundfile, librosa
+#   git, build-essential   - building source-only wheels
+#   pkg-config + libav*-dev - required to compile PyAV (a faster-whisper dep)
 RUN apt-get update && apt-get install -y --no-install-recommends \
-        ffmpeg git libsndfile1 build-essential \
+        ffmpeg git libsndfile1 build-essential pkg-config \
+        libavformat-dev libavcodec-dev libavdevice-dev libavutil-dev \
+        libavfilter-dev libswscale-dev libswresample-dev \
     && rm -rf /var/lib/apt/lists/*
 
 # Install the CPU build of PyTorch first so the heavy CUDA wheel is never