merge #518 into opencontainers/umoci:main #1138

Job	Run time
release	1m 2s
macos	1m 14s
linux-ci-image	2m 51s
validate	1m 14s
unit	1m 36s
integration (opensuse/leap:latest)	8m 53s
integration (centos:latest)	13m 24s
integration (debian:latest)	10m 4s
integration (ubuntu:latest)	7m 14s
integration (fedora:latest)	11m 30s
coverage	39s
	59m 41s

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