cmd/xb: add ARM images to valid layers lists

This change allows ARM based docker images to be built by xb.

For golang/go#53851

Change-Id: I5a2fd92b4a93957551e1f7cb9f46871de54fbb1d
Reviewed-on: https://go-review.googlesource.com/c/build/+/453657
Run-TryBot: Carlos Amedee <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

Author: cagedmantis

cmd/xb/xb.go

@@ -128,6 +128,8 @@ func runDocker() {
 		for _, layer := range layers {
 			if strings.HasPrefix(layer, "golang:") ||
 				strings.HasPrefix(layer, "debian:") ||
+				strings.HasPrefix(layer, "arm32v6/debian:") ||
+				strings.HasPrefix(layer, "arm64v8/debian:") ||
 				strings.HasPrefix(layer, "alpine:") ||
 				strings.HasPrefix(layer, "fedora:") {
 				continue