Add libgtk-3-0 to Dockerfile for Audiveris JNA dependency

Audiveris loads GTK3 via JNA even in headless batch mode.
Without libgtk-3-0, all OMR conversions fail with
UnsatisfiedLinkError: libgtk-3.so not found.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Dockerfile

@@ -29,6 +29,7 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
         tesseract-ocr-eng \
         libgl1 \
         libglib2.0-0 \
+        libgtk-3-0 \
     && rm -rf /var/lib/apt/lists/*
 
 # Copy built Audiveris