Fix Docker build warning for undefined TARGETARCH variable

contrib/build-linux/appimage/Dockerfile

@@ -2,9 +2,9 @@
 # from https://docs.appimage.org/introduction/concepts.html :
 # "[AppImages] should be built on the oldest possible system, allowing them to run on newer system[s]"
 
-ARG TARGETARCH=x86_64
 FROM debian:bullseye@sha256:cf48c31af360e1c0a0aedd33aae4d928b68c2cdf093f1612650eb1ff434d1c34
 
+ARG TARGETARCH=x86_64
 ENV LC_ALL=C.UTF-8 LANG=C.UTF-8
 ENV DEBIAN_FRONTEND=noninteractive
 ENV TARGETARCH=${TARGETARCH}