Require dockerfile to be specified relative to cargo workspace root #4290

Job	Run time
cargo-deny	15s
fmt	8s
clippy (macos-latest)	22s
clippy (ubuntu-latest)	22s
clippy (windows-latest)	45s
generate-matrix	15s
check	31s
shellcheck	8s
test (macos-latest)	57s
test (ubuntu-latest)	55s
test (windows-latest)	2m 35s
bisect	3m 56s
remote	1m 54s
docker-in-docker	3m 48s
target (${{ matrix.pretty }},${{ matrix.os }})	0s
foreign	2m 25s
podman	1m 44s
publish	0s
conclusion	1s
Coverage	13s
	21m 14s

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