[COST-5214] Move TARGETARCH declaration to the top of the Dockerfile (#5195)

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.

Update Jenkinsfile to use RHEL 8

Unfortunately this breaks the image build for Docker. I'll fix that in a followup PR.

Author: samdoran

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

Jenkinsfile

@@ -22,7 +22,7 @@ def secrets = [
 def configuration = [vaultUrl: params.VAULT_ADDRESS, vaultCredentialId: params.VAULT_CREDS_ID, engineVersion: 1]
 
 pipeline {
-    agent { label 'insights' }
+    agent { label 'rhel8' }
     options {
         timestamps()
     }