Move TARGETARCH declaration to the top of the Dockerfile

There is a bug in podman where this is only used correctly for the
multi-stage build if it is defined as the first line.

Dockerfile

@@ -1,3 +1,5 @@
+ARG TARGETARCH
+
 FROM registry.access.redhat.com/ubi8/ubi-minimal:latest AS base
 
 USER root
@@ -61,8 +63,6 @@ RUN ldconfig
 # No intermetiate steps for x86_64, but declare it so it can be used for the final image
 FROM --platform=amd64 base AS stage-amd64
 
-ARG TARGETARCH
-
 FROM stage-${TARGETARCH} AS final
 # PIPENV_DEV is set to true in the docker-compose allowing
 # local builds to install the dev dependencies