unix/linux: update Dockerfile to Linux Kernel 4.20

Change-Id: Iadc92b5afd18a5c1df0bc42b0d6f52f0b768b47a
Reviewed-on: https://go-review.googlesource.com/c/155979
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/unix/ztypes_linux_mipsle.go b/unix/ztypes_linux_mipsle.go
index dac6168..41cb148 100644
--- a/unix/ztypes_linux_mipsle.go
+++ b/unix/ztypes_linux_mipsle.go
@@ -822,6 +822,8 @@
 	Cpu_scaled_run_real_total uint64
 	Freepages_count           uint64
 	Freepages_delay_total     uint64
+	Thrashing_count           uint64
+	Thrashing_delay_total     uint64
 }
 
 const (