Make another docker layer reusable

By moving the apt-get layer before the copies, the apt-get layer can be reused with each build, reducing upload/download needed during updates.

Just a small optimization.

Dockerfile.arm

@@ -16,9 +16,9 @@ RUN export DOTNET_CLI_TELEMETRY_OPTOUT=1 \
 
 
 FROM microsoft/dotnet:${DOTNET_VERSION}-runtime-stretch-slim-arm32v7
-COPY --from=builder /jellyfin /jellyfin
-EXPOSE 8096
 RUN apt-get update \
  && apt-get install -y ffmpeg
+COPY --from=builder /jellyfin /jellyfin
+EXPOSE 8096
 VOLUME /config /media
 ENTRYPOINT dotnet /jellyfin/jellyfin.dll -programdata /config