Missing '/wait-for-manager.sh' script in alpine version

[kgonia]

Steps to reproduce:

docker run --rm -it citusdata/citus:9.4.2-alpine ls

[DilithiumNitrate]

Still the case in citusdata/citus:11.1.5-alpine even though the Dockerfile has seen numerous updates. The script itself doesn't work inside Alpine because of the lack of gosu, but as per https://github.com/tianon/gosu/blob/master/INSTALL.md, there is a trivial fix: installing su-exec and using that instead of gosu on Alpine.