cmd/rundockerbuildlet: set --security-opt=seccomp=unconfined in containers
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 <60c6d277a8bd81de7fdde19201bf9c58a3df08f4@golang.org>
Run-TryBot: Alexander Rakoczy <60c6d277a8bd81de7fdde19201bf9c58a3df08f4@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/build,golang/build,golang/build,golang/build