unix: update Dockerfile to Linux 5.8

Change-Id: If6643e7e092c5d482f9cefe838640ebcec22188e
Reviewed-on: https://go-review.googlesource.com/c/sys/+/246417
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matt Layher <mdlayher@gmail.com>
diff --git a/unix/zsysnum_linux_386.go b/unix/zsysnum_linux_386.go
index 54559a8..a597e06 100644
--- a/unix/zsysnum_linux_386.go
+++ b/unix/zsysnum_linux_386.go
@@ -433,4 +433,5 @@
 	SYS_CLONE3                       = 435
 	SYS_OPENAT2                      = 437
 	SYS_PIDFD_GETFD                  = 438
+	SYS_FACCESSAT2                   = 439
 )