document how to build under alpine #6400

Job	Run time
cross-i386	1m 28s
cross-i386 (runc_nodmz)	1m 30s
test (ubuntu-20.04, 1.21.x, rootless, -race)	7m 31s
test (ubuntu-20.04, 1.21.x, rootless)	6m 39s
test (ubuntu-20.04, 1.21.x, -race)	17m 35s
test (ubuntu-20.04, 1.21.x)	7m 1s
test (ubuntu-20.04, 1.22.x, rootless, -race)	7m 20s
test (ubuntu-20.04, 1.22.x, rootless)	7m 34s
test (ubuntu-20.04, 1.22.x, -race)	8m 3s
test (ubuntu-20.04, 1.22.x)	7m 11s
test (ubuntu-20.04, 1.22.x, criu-dev)	8m 15s
test (ubuntu-24.04, 1.21.x, rootless, -race)	9m 18s
test (ubuntu-24.04, 1.21.x, rootless)	9m 0s
test (ubuntu-24.04, 1.21.x, -race)	8m 7s
test (ubuntu-24.04, 1.21.x)	7m 14s
test (ubuntu-24.04, 1.22.x, rootless, -race)	9m 9s
test (ubuntu-24.04, 1.22.x, rootless)	9m 11s
test (ubuntu-24.04, 1.22.x, -race)	7m 58s
test (ubuntu-24.04, 1.22.x)	7m 22s
test (ubuntu-24.04, 1.22.x, runc_nodmz)	7m 17s
test (ubuntu-24.04, 1.22.x, criu-dev)	7m 52s
test (actuated-arm64-6cpu-8gb, 1.22.x, rootless)	15m 23s
test (actuated-arm64-6cpu-8gb, 1.22.x)	11m 46s
	3h 9m 44s

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