unix/linux: update Dockerfile to Kernel 5.15

Change-Id: I032465fbaecd49ffb6048577cd2f3fcb3d8e11d0
Reviewed-on: https://go-review.googlesource.com/c/sys/+/359678
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
diff --git a/unix/zsysnum_linux_386.go b/unix/zsysnum_linux_386.go
index aa7ce85..31847d2 100644
--- a/unix/zsysnum_linux_386.go
+++ b/unix/zsysnum_linux_386.go
@@ -444,4 +444,5 @@
 	SYS_LANDLOCK_ADD_RULE            = 445
 	SYS_LANDLOCK_RESTRICT_SELF       = 446
 	SYS_MEMFD_SECRET                 = 447
+	SYS_PROCESS_MRELEASE             = 448
 )