app-alternatives/gzip: Add from Gentoo

It's from Gentoo commit d06f9f0d0d1ddb309e4a028a8f0f44a5247d85e9.

sdk_container/src/third_party/portage-stable/app-alternatives/gzip/gzip-0.ebuild

@@ -0,0 +1,44 @@
+# Copyright 2022 Gentoo Authors
+# Distributed under the terms of the GNU General Public License v2
+
+EAPI=8
+
+ALTERNATIVES=(
+	"reference:>=app-arch/gzip-1.12-r3"
+	"pigz:app-arch/pigz[-symlink(-)]"
+)
+
+inherit app-alternatives
+
+DESCRIPTION="gzip symlinks"
+KEYWORDS="~alpha amd64 arm arm64 hppa ~ia64 ~loong ~m68k ~mips ppc ppc64 ~riscv ~s390 sparc x86 ~x64-cygwin ~amd64-linux ~x86-linux ~ppc-macos ~x64-macos ~sparc-solaris ~sparc64-solaris ~x64-solaris ~x86-solaris"
+IUSE="split-usr"
+
+RDEPEND="
+	!<app-arch/gzip-1.12-r3
+	!app-arch/pigz[symlink(-)]
+"
+
+src_install() {
+	local usr_prefix=
+	use split-usr && usr_prefix=../usr/bin/
+
+	case $(get_alternative) in
+		pigz)
+			dosym "${usr_prefix}pigz" /bin/gzip
+			dosym gzip /bin/gunzip
+			dosym gzip /bin/zcat
+			newman - gzip.1 <<<".so pigz.1"
+			;;
+		reference)
+			dosym gzip-reference /bin/gzip
+			# gzip uses shell wrappers rather than argv[0]
+			dosym gunzip-reference /bin/gunzip
+			dosym zcat-reference /bin/zcat
+			newman - gzip.1 <<<".so gzip-reference.1"
+			;;
+	esac
+
+	newman - gunzip.1 <<<".so gzip.1"
+	newman - zcat.1 <<<".so gzip.1"
+}

sdk_container/src/third_party/portage-stable/app-alternatives/gzip/metadata.xml

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+<?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>
+		<name>Gentoo Base System</name>
+	</maintainer>
+	<maintainer type="person">
+		<email>mgorny@gentoo.org</email>
+		<name>Michał Górny</name>
+	</maintainer>
+	<use>
+		<flag name="pigz">
+			Symlink to <pkg>app-arch/pigz</pkg>.
+		</flag>
+		<flag name="reference">
+			Symlink to <pkg>app-arch/gzip</pkg>.
+		</flag>
+	</use>
+</pkgmetadata>