Fix default build arg

Dockerfile

@@ -1,4 +1,5 @@
 ARG DOTNET_VERSION=2.2
+ARG FFMPEG_VERSION=latest
 
 FROM mcr.microsoft.com/dotnet/core/sdk:${DOTNET_VERSION} as builder
 WORKDIR /repo
@@ -7,7 +8,6 @@ ENV DOTNET_CLI_TELEMETRY_OPTOUT=1
 RUN bash -c "source deployment/common.build.sh && \
     build_jellyfin Jellyfin.Server Release linux-x64 /jellyfin"
 
-ARG FFMPEG_VERSION=latest
 FROM jellyfin/ffmpeg:${FFMPEG_VERSION} as ffmpeg
 FROM mcr.microsoft.com/dotnet/core/runtime:${DOTNET_VERSION}
 # libfontconfig1 is required for Skia