Create miopen_cache dir in Dockerfile before volume mount

MIOpen crashes with SQLite 'unable to open database file' when the
directory doesn't exist at container start. mkdir + chmod 777 ensures
the path is present and writable before the named volume overlays it.

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

Dockerfile.rocm

@@ -41,8 +41,10 @@ COPY engine.py config.py wyoming_handler.py wyoming_voices.py main.py ./
 # Default config (can be overridden by volume mount)
 COPY config.yaml .
 
-# Create default directories
-RUN mkdir -p voices reference_audio hf_cache
+# Create default directories (miopen_cache must exist before the volume mounts
+# so MIOpen can open its SQLite benchmark database without crashing)
+RUN mkdir -p voices reference_audio hf_cache miopen_cache && \
+    chmod 777 miopen_cache
 
 EXPOSE 10200