Docs: Change latest to main in dockerfile example #4145

Job	Run time
shellcheck	8s
clippy (macos-latest)	44s
clippy (ubuntu-latest)	28s
cargo-deny	20s
clippy (windows-latest)	1m 9s
check	20s
fmt	18s
generate-matrix	22s
test (macos-latest)	1m 29s
test (ubuntu-latest)	1m 13s
test (windows-latest)	2m 11s
foreign	2m 33s
bisect	5m 53s
docker-in-docker	4m 9s
target (${{ matrix.pretty }},${{ matrix.os }})	0s
podman	2m 8s
remote	2m 18s
publish	0s
conclusion	1s
Coverage	20s
	26m 4s

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