Replace the bare /init ENTRYPOINT with entrypoint-dispatch.sh: exec
/init + main-wrapper when the image owns PID 1, fall back to a direct
stage2 bootstrap (with the s6 helper PATH restored) on wrapped runtimes
where s6-overlay-suexec would abort with 'can only run as pid 1'
(Fly Machines, docker run --init, podman/FreeBSD setups).
Cherry-picked from PR #43763 by @konsisumer, conflicts with current
main resolved (tests/test_dockerfile_tini_compat_shim.py was moved to
tests/docker/, container_boot argv tests were reshaped upstream).
Fixes#38349
