overlay app-admin/sudo: Keep /etc/sudoers.d directory

It seems to be randomly kept or removed during installation, and we
need the directory when creating the SDK container.

sdk_container/src/third_party/coreos-overlay/app-admin/sudo/sudo-1.9.15_p5.ebuild

@@ -240,6 +240,11 @@ src_install() {
 	# Flatcar: Remove sudo.conf as it is shipped via baselayout
 	rm "${ED}/etc/sudo.conf" || die
 
+	# Flatcar: Build system installs /etc/sudoers.d, let's make
+	# sure we keep having it.
+	#
+	# Upstream PR: https://github.com/gentoo/gentoo/pull/37397
+	keepdir /etc/sudoers.d
 }
 
 pkg_postinst() {