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

ui/build/Android.bp

@@ -34,15 +34,16 @@ bootstrap_go_package {
     deps: [
         "blueprint",
         "blueprint-bootstrap",
+        "blueprint-microfactory",
+        "soong-finder",
+        "soong-remoteexec",
+        "soong-shared",
         "soong-ui-build-paths",
         "soong-ui-logger",
         "soong-ui-metrics",
         "soong-ui-status",
         "soong-ui-terminal",
         "soong-ui-tracer",
-        "soong-shared",
-        "soong-finder",
-        "blueprint-microfactory",
     ],
     srcs: [
         "bazel.go",

ui/build/rbe.go

@@ -21,6 +21,7 @@ import (
 	"runtime"
 	"strings"
 
+	"android/soong/remoteexec"
 	"android/soong/ui/metrics"
 )
 
@@ -54,11 +55,12 @@ func rbeCommand(ctx Context, config Config, rbeCmd string) string {
 
 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()))