acct-group/shadow: Add from Gentoo

It's from Gentoo commit 5b84adec2b5de578a3fb055b5aabc2160676dda0.

Signed-off-by: Krzesimir Nowak <knowak@microsoft.com>

sdk_container/src/third_party/portage-stable/acct-group/shadow/metadata.xml

@@ -0,0 +1,7 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE pkgmetadata SYSTEM "https://www.gentoo.org/dtd/metadata.dtd">
+<pkgmetadata>
+	<maintainer type="project">
+		<email>base-system@gentoo.org</email>
+	</maintainer>
+</pkgmetadata>

sdk_container/src/third_party/portage-stable/acct-group/shadow/shadow-0.ebuild

@@ -0,0 +1,24 @@
+# Copyright 2025 Gentoo Authors
+# Distributed under the terms of the GNU General Public License v2
+
+EAPI=8
+
+inherit acct-group user-info
+
+ACCT_GROUP_ID=42
+
+pkg_postinst() {
+	# Look up the gid in ${EROOT}/etc/group.
+	# It may differ from the gid in /etc/group.
+	local gid=$(egetent group shadow | cut -d: -f3)
+	if [[ -z ${gid} ]]; then
+		eerror "Unable to determine id for shadow group"
+		return
+	fi
+	local db
+	for db in gshadow shadow; do
+		[[ -e ${EROOT}/etc/${db} ]] || continue
+		chgrp "${gid}" "${EROOT}/etc/${db}"
+		chmod g+r "${EROOT}/etc/${db}"
+	done
+}