cmd/rundockerbuildlet: set --security-opt=seccomp=unconfined in containers

This fixes race tests; the thread sanitizer needs to check its
personality, which seccomp defaults prevent apparently.

Updates golang/go#35547 (needs to be deployed first, then bug can be closed)

Change-Id: I8b87618f63ef2b7a75b72290098c09bf04298d86
Reviewed-on: https://go-review.googlesource.com/c/build/+/214919
Reviewed-by: Alexander Rakoczy <alex@golang.org>
Run-TryBot: Alexander Rakoczy <alex@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
diff --git a/cmd/rundockerbuildlet/rundockerbuildlet.go b/cmd/rundockerbuildlet/rundockerbuildlet.go
index 4d0ff12..c437b08 100644
--- a/cmd/rundockerbuildlet/rundockerbuildlet.go
+++ b/cmd/rundockerbuildlet/rundockerbuildlet.go
@@ -187,6 +187,7 @@
 			"--name="+name,
 			"-v", filepath.Dir(keyFile)+":/buildkey/",
 			"-e", "HOSTNAME="+name,
+			"--security-opt=seccomp=unconfined", // Issue 35547
 			"--tmpfs=/workdir:rw,exec")
 		if *memory != "" {
 			cmd.Args = append(cmd.Args, "--memory="+*memory)