missing this definition from libc/kernel headers unistd.h:

sandbox.c:184:40: error: '__NR_userfaultfd' undeclared (first use in
this function)

Enable namespaces

See merge request ai3/suexec-sandbox!1

New namespaces are not allowed within unprivileged docker containers,
thus skip it.

Read per-user configuration files from
/etc/apache2/suexec-sandbox.d/UID, that can override directives from
the global (default) config.