[tmp] Use amd64 test image

This is a workaround because official images were pushed with multi-arch
support recently and the current Pull code is not accounting for this.

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

client_unix_test.go

@@ -33,6 +33,7 @@ func init() {
 	case "s390x":
 		testImage = "docker.io/s390x/alpine:latest"
 	default:
-		testImage = "docker.io/library/alpine:latest"
+		// FIXME: change this back after multiplatform support is added to pull
+		testImage = "docker.io/amd64/alpine:latest"
 	}
 }