optional cache-to/from #104

Job	Run time
build-docker-normal / build-docker-image (amd64)	37s
build-docker-normal / build-docker-image (ppc64le)	31s
build-docker-normal / build-docker-image (riscv64)	33s
build-docker-normal / build-docker-image (arm/v7)	25s
build-docker-normal / build-docker-image (s390x)	30s
build-docker-normal / build-docker-image (arm64)	27s
build-docker-normal / merge	17s
build-docker-normal / cleanup-docker	13s
build-docker-minimal / build-docker-image (amd64)	28s
build-docker-minimal / build-docker-image (arm64)	39s
build-docker-minimal / build-docker-image (arm/v7)	32s
build-docker-minimal / build-docker-image (riscv64)	23s
build-docker-minimal / build-docker-image (ppc64le)	27s
build-docker-minimal / build-docker-image (s390x)	25s
build-docker-minimal / merge	23s
build-docker-minimal / cleanup-docker	13s
cleanup-docker-cache	10s
	7m 13s

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