docs: Document `to-disk --via-loopback` #694

Job	Run time
build-c9s	3m 28s
build-fedora	4m 31s
Build skopeo git main for ubuntu	2m 3s
cargo-deny	28s
tests	5m 45s
Container testing	48s
Privileged testing	33s
Test install-alongside	54s
	18m 30s

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