Specify platform linux/amd64 for building image

Using ci-test.sh with unset USE_RHTAP_IMAGES environment variable fails
on platforms different from linux/amd64.
`make build-image` builds an image for the platform it detects. It can
be different from linux/amd64 which the CI providers require.

Signed-off-by: Tomáš Nevrlka <[email protected]>

Author: tnevrlka

Makefile

@@ -159,7 +159,7 @@ push-image:
 
 .PHONY: build-image
 build-image:
-	podman build $(if $(NOCACHE),--no-cache) -f Dockerfile -t $(floating-tag)
+	podman build $(if $(NOCACHE),--no-cache) -f Dockerfile -t $(floating-tag) --platform linux/amd64
 	podman tag $(floating-tag) $(unique-tag)
 
 .PHONY: run-image