Use setpriv instead of gosu for dropping privileges in the entrypoint (redis#435)

* Use setpriv instead of gosu to drop privileges

Changes:

setpriv is used instead of gosu with the following flags:

* Set reuid and regid to redis user and group
* Clear all supplementary groups
* Set bouding capabilities to an empty list
* Enable no-new-privs bit
* Set securebit to exclude regaining capabilities

Author: Peter-Sh