Export default RBE_platform

Export it so that we have one source of truth,
which can be used by the called tools.

Test: Use the exported value in ART build.
Change-Id: Iaa45b1780ffae11e09790e193ca2bd01e2b732a8
diff --git a/ui/build/rbe.go b/ui/build/rbe.go
index 82fc15f..6231e52 100644
--- a/ui/build/rbe.go
+++ b/ui/build/rbe.go
@@ -21,6 +21,7 @@
 	"runtime"
 	"strings"
 
+	"android/soong/remoteexec"
 	"android/soong/ui/metrics"
 )
 
@@ -54,11 +55,12 @@
 
 func getRBEVars(ctx Context, config Config) map[string]string {
 	vars := map[string]string{
-		"RBE_log_dir":    config.rbeProxyLogsDir(),
-		"RBE_re_proxy":   config.rbeReproxy(),
-		"RBE_exec_root":  config.rbeExecRoot(),
-		"RBE_output_dir": config.rbeProxyLogsDir(),
+		"RBE_log_dir":       config.rbeProxyLogsDir(),
+		"RBE_re_proxy":      config.rbeReproxy(),
+		"RBE_exec_root":     config.rbeExecRoot(),
+		"RBE_output_dir":    config.rbeProxyLogsDir(),
 		"RBE_proxy_log_dir": config.rbeProxyLogsDir(),
+		"RBE_platform":      "container-image=" + remoteexec.DefaultImage,
 	}
 	if config.StartRBE() {
 		name, err := config.rbeSockAddr(absPath(ctx, config.TempDir()))