unix: update Dockerfile to Linux 4.18, glibc 2.28 and Go 1.11beta3

Including linux/stat.h in mkerrors.sh is no longer necessary because the
statx types and constants were added to the glibc headers.

Change-Id: I643c35fd2da76dd2c15015556658d7fa862249c4
Reviewed-on: https://go-review.googlesource.com/129100
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

unix/zsysnum_linux_arm.go

@@ -359,4 +359,5 @@ const (
 	SYS_PKEY_ALLOC             = 395
 	SYS_PKEY_FREE              = 396
 	SYS_STATX                  = 397
+	SYS_RSEQ                   = 398
 )