add Makefile: multi-arch build-push (amd64+arm64) for the mixed k8s cluster

The single-arch image crashed with 'exec format error' on arm64 nodes. This
codifies the podman multi-arch manifest build so future pushes stay multi-arch.

Makefile

@@ -0,0 +1,16 @@
+IMAGE := edenic-exporter
+REGISTRY := mysources.co.uk
+REGISTRY_IMAGE := $(REGISTRY)/michal/$(IMAGE)
+# The k8s cluster is mixed amd64+arm64 — the registry image MUST be a multi-arch
+# manifest list, or pods on the "other" arch crash with `exec format error`.
+PLATFORMS := linux/amd64,linux/arm64
+MANIFEST := edenic-exporter-mm
+
+.PHONY: build-push
+# Log in once first (GITEA_TOKEN):
+#   echo "$$GITEA_TOKEN" | podman login mysources.co.uk -u michal --password-stdin
+build-push:
+	-podman manifest rm $(MANIFEST) 2>/dev/null
+	podman build --platform $(PLATFORMS) --manifest $(MANIFEST) .
+	podman manifest push --all $(MANIFEST) docker://$(REGISTRY_IMAGE):latest
+	podman manifest inspect docker://$(REGISTRY_IMAGE):latest | grep architecture