chroot doesn't matter: in 2002 I wrote in the French MISC magazine 11 ways to break out of a chroot jail. One of them applies here: chroot doesn't matter if you have CAP_SETUID, in fact CAP_SETUID is basically equivalent to CAP_SYS_PTRACE. If i can change to any UID, then I can effectively ptrace any process (including those running outside of the chroot) giving me full control of the host system.