The namespace init process
The first process created in a new namespace (i.e., the process
created using clone(2) with the CLONE_NEWPID flag, or the first child
created by a process after a call to unshare(2) using the
CLONE_NEWPID flag) has the PID 1, and is the “init” process for the
namespace (see init(1)). A child process that is orphaned within the
namespace will be reparented to this process rather than init(1)
(unless one of the ancestors of the child in the same PID namespace
employed the prctl(2) PR_SET_CHILD_SUBREAPER command to mark itself
as the reaper of orphaned descendant processes).