GetAdditionalGroupsForUser returns a list of gids that userid is associated with.

GetGroup returns the gid by looking it up in the /etc/group file groupspec format [ group | gid ].

GetUser will return the uid, gid of the user specified in the userspec it will use the /etc/passwd and /etc/group files inside of the rootdir to return this information.

LookupUIDInContainer returns username and gid associated with a UID in a container it will use the /etc/passwd files inside of the rootdir to return this information.

ErrNoSuchUser indicates that the user provided by the caller does not exist in /etc/passws.