all: Use Linux container CPU quota (fixes #9357, fixes #9435) (#9436)

Go is not cgroup aware and by default will set GOMAXPROCS to the number
of available threads, regardless of whether it is within the allocated
quota. This behaviour causes high amount of CPU throttling and degraded
application performance.

build.go

@@ -34,6 +34,7 @@ import (
 	"time"
 
 	buildpkg "github.com/syncthing/syncthing/lib/build"
+	_ "go.uber.org/automaxprocs"
 )
 
 var (