build

refactor: use portage Docker image to reduce infra load #1407

Job	Run time
386:tcp image	1m 25s
386:ssh image	1m 18s
amd64:tcp image	1m 16s
amd64:ssh image	1m 11s
arm/v6:tcp image	2m 43s
arm/v6:ssh image	2m 41s
arm/v7:tcp image	2m 40s
arm/v7:ssh image	2m 42s
arm64:tcp image	2m 40s
arm64:ssh image	2m 41s
ppc64le:tcp image	2m 39s
ppc64le:ssh image	2m 41s
${{ matrix.manifest.registry }}:${{ matrix.manifest.tag }} manifest	1s
	26m 38s

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