app-containers/docker-buildx: Sync with Gentoo

It's from Gentoo commit 8b5b838a550e0d1339dd04886be606a4aea8a3b3.

sdk_container/src/third_party/portage-stable/app-containers/docker-buildx/Manifest

@@ -2,3 +2,4 @@ DIST docker-buildx-0.11.2.tar.gz 10342565 BLAKE2B 1e034e9a07f36b46096f8f035869ad
 DIST docker-buildx-0.14.0.tar.gz 11120356 BLAKE2B 01fec6b29665088a4778cb6a926f071c1beacb22d7a50593d55afff1225d0e74d083a82eaab4ae96ffee8a6f653ce048fd81b3410e7113bc29da210d77042b64 SHA512 bc09f29096cfd08c60efd8fca7976a1db896f0ed08d33dfc64dfca2c6487a1dc1722b3fe80b6a175ebebeacf8b460bcc214806d0c7a637f17ed5d09793d8c9d8
 DIST docker-buildx-0.19.1.tar.gz 11691424 BLAKE2B 40cc5e68eb85e8693bc6cd7bf91fe3a6300644d9934a3ff84b89c79dba31d475dce5ac4366075127391c5230905591a83bd59c762cf9f9f4bde0931834633473 SHA512 cc811eb93910bfb5a4668cc3dc59ab6c13a1308c0fa49707a81d1d733e834d209677502786e951ce3c8f098c387b559054dd3cd9933edafbecae610d1ef321e6
 DIST docker-buildx-0.20.1.tar.gz 12049522 BLAKE2B cc370f8a6bc82d9358cb59f99645447e83a4c84c656ae75409a504d0e0a973027791129d6eeb55827eb98904300d04c34685be993309e80f547514b76566fb2e SHA512 75c47204a2f11773ed978678735637fb8248ad87bb745e0b878ad7db161dea4d000dbfb98c83b03225efed126cc04b31d3a9ca500f8d3ac414620c7f980fa9cc
+DIST docker-buildx-0.21.2.tar.gz 13608048 BLAKE2B 8229fe6d8d1b2b9f35213ae0ae879d90d7f96ca4688c787e7dcbb88e0aaa4444519c4e5fca51acf9e9d6f2bf697adf74b2c2d8440eec1660dbf10496eb76c243 SHA512 a90a9af1a9b986f99a08f3e53ca06cfac64378a92ab887653d3aa9e1538ef87fec4fbd719c5087782cf7dc5eb8845c9c0786605e7386d759c0e33ef5f121fbba

sdk_container/src/third_party/portage-stable/app-containers/docker-buildx/docker-buildx-0.21.2.ebuild

@@ -0,0 +1,56 @@
+# Copyright 1999-2025 Gentoo Authors
+# Distributed under the terms of the GNU General Public License v2
+
+EAPI=8
+
+inherit go-module
+
+DESCRIPTION="Docker CLI plugin for extended build capabilities with BuildKit"
+HOMEPAGE="https://github.com/docker/buildx"
+
+if [[ ${PV} == 9999 ]]; then
+	inherit git-r3
+	EGIT_REPO_URI="https://github.com/docker/buildx.git"
+else
+	SRC_URI="https://github.com/docker/buildx/archive/refs/tags/v${PV}.tar.gz -> ${P}.tar.gz"
+	KEYWORDS="~amd64 ~arm ~arm64 ~ppc64 ~riscv ~x86"
+	S=${WORKDIR}/${P#docker-}
+fi
+
+LICENSE="Apache-2.0"
+# Dependent licenses
+LICENSE+=" Apache-2.0 BSD BSD-2 ISC MIT MPL-2.0"
+SLOT="0"
+
+RDEPEND="app-containers/docker-cli"
+
+src_compile() {
+	local _buildx_r='github.com/docker/buildx'
+	local version=${PV}
+	if [[ ${PV} == 9999 ]]; then
+		version="$(git rev-parse --short HEAD)"
+	fi
+	local go_ldflags=(
+		"-linkmode=external"
+		-X "${_buildx_r}/version.Version=${version}"
+		-X "${_buildx_r}/version.Revision=$(date -u +%FT%T%z)"
+		-X "${_buildx_r}/version.Package=${_buildx_r}"
+	)
+	ego build -o docker-buildx -ldflags "${go_ldflags[*]}" ./cmd/buildx
+}
+
+src_test() {
+	# TestGit can't work in a source tarball; TestReadTargets fails seemingly due to parallelism.
+	if [[ ${PV} == 9999 ]]; then
+		ego test ./... -skip "TestReadTargets|TestIntegration"
+	else
+		ego test ./... -skip "TestGit|TestReadTargets|TestIntegration"
+	fi
+}
+
+src_install() {
+	exeinto /usr/libexec/docker/cli-plugins
+	doexe docker-buildx
+
+	einstalldocs
+}