helps if the arg is within that bit of the dockerfile

Dockerfile

@@ -1,5 +1,4 @@
 ARG DOTNET_VERSION=2
-ARG FFMPEG_URL=https://www.johnvansickle.com/ffmpeg/old-releases/ffmpeg-4.0.3-64bit-static.tar.xz
 
 FROM microsoft/dotnet:${DOTNET_VERSION}-sdk as builder
 WORKDIR /repo
@@ -14,6 +13,7 @@ EXPOSE 8096
 
 VOLUME /config /media
 
+ARG FFMPEG_URL=https://www.johnvansickle.com/ffmpeg/old-releases/ffmpeg-4.0.3-64bit-static.tar.xz
 RUN apt update \
  && apt install -y xz-utils \
  && curl ${FFMPEG_URL} | tar Jxf - -C /usr/bin --wildcards --strip-components=1 ffmpeg*/ffmpeg ffmpeg*/ffprobe \