fix(ffmpeg): use generic Sans font and add detailed font debug logs

ffmpeg-service/Dockerfile

@@ -14,7 +14,8 @@ RUN wget -q https://johnvansickle.com/ffmpeg/releases/ffmpeg-release-amd64-stati
     && tar xf ffmpeg-release-amd64-static.tar.xz \
     && mv ffmpeg-*-amd64-static/ffmpeg /usr/local/bin/ \
     && mv ffmpeg-*-amd64-static/ffprobe /usr/local/bin/ \
-    && rm -rf ffmpeg-*
+    && rm -rf ffmpeg-* \
+    && fc-cache -fv
 
 WORKDIR /app