Use VAAPI-enabled ffmpeg

Dockerfile

@@ -10,7 +10,7 @@ RUN export DOTNET_CLI_TELEMETRY_OPTOUT=1 \
     --output /jellyfin \
     Jellyfin.Server
 
-FROM jrottenberg/ffmpeg:4.0-scratch as ffmpeg
+FROM jrottenberg/ffmpeg:4.0-vaapi as ffmpeg
 FROM microsoft/dotnet:${DOTNET_VERSION}-runtime
 # libfontconfig1 is required for Skia
 RUN apt-get update \