merge #503 into opencontainers/umoci:main #1146

Job	Run time
linux-ci-image	2m 56s
macos	1m 25s
release	1m 2s
validate	27s
unit	1m 22s
integration (opensuse/leap:latest)	8m 40s
integration (centos:latest)	13m 42s
integration (debian:latest)	9m 32s
integration (ubuntu:latest)	6m 59s
integration (fedora:latest)	11m 5s
coverage	29s
	57m 39s

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