imagebuildah.StageExecutor.prepare(): log the --platform flag

If FROM was used with a --platform flag, then the imagebuilder.Builder
will have its Platform field set, and we should include it when logging
the instruction.

Signed-off-by: Nalin Dahyabhai <nalin@redhat.com>

imagebuildah/stage_executor.go

@@ -860,6 +860,9 @@ func (s *StageExecutor) prepare(ctx context.Context, from string, initializeIBCo
 		from = base
 	}
 	displayFrom := from
+	if ib.Platform != "" {
+		displayFrom = "--platform=" + ib.Platform + " " + displayFrom
+	}
 
 	// stage.Name will be a numeric string for all stages without an "AS" clause
 	asImageName := stage.Name

tests/bud.bats

@@ -6838,3 +6838,11 @@ _EOF
   run_buildah build $WITH_POLICY_JSON --no-cache --isolation chroot --secret id=MYSECRET -t test -f $contextdir/Dockerfile
   expect_output --substring "SOMESECRETDATA"
 }
+
+@test "build-logs-from-platform" {
+  run_buildah info --format '{{.host.os}}/{{.host.arch}}{{if .host.variant}}/{{.host.variant}}{{ end }}'
+  local platform="$output"
+  echo FROM --platform=$platform busybox > ${TEST_SCRATCH_DIR}/Containerfile
+  run_buildah build ${TEST_SCRATCH_DIR}
+  expect_output --substring "\-\-platform=$platform"
+}