playground: fix the build for gVisor

Exec permissions need to be granted on /tmpfs on Docker in order for
binaries downloaded there to be executable. Additionally, newer versions
of gVisor only create the __runsc_containers__ file in a multi-container
environment, which is not the case for testing.

Updates golang/go#25224

Change-Id: I272535558d952e423f75ba9770a491472e9c8eff
Reviewed-on: https://go-review.googlesource.com/c/playground/+/227349
Reviewed-by: Carlos Amedee <carlos@golang.org>

sandbox/sandbox.go

diff --git a/sandbox/sandbox.go b/sandbox/sandbox.go
index 41f1f9b..9b8bcba 100644
--- a/sandbox/sandbox.go
+++ b/sandbox/sandbox.go
@@ -358,7 +358,7 @@
 	cmd := exec.Command("docker", "run",
 		"--name="+name,
 		"--rm",
-		"--tmpfs=/tmpfs",
+		"--tmpfs=/tmpfs:exec",
 		"-i", // read stdin
 
 		"--runtime=runsc",

tests.go

diff --git a/tests.go b/tests.go
index 4022b6a..0f3cda4 100644
--- a/tests.go
+++ b/tests.go
@@ -264,7 +264,6 @@
 			}
 			for _, expect := range []string{
 				"/.dockerenv",
-				"/__runsc_containers__",
 				"/etc/hostname",
 				"/dev/zero",
 				"/lib/ld-linux-x86-64.so.2",