This is fixed upstream. The file appears in user namespaces at least from kernels 4.13 onwards but it must be turned on way earlier.
Author: Nikolay Borisov firstname.lastname@example.org
Date: Thu Jan 7 16:38:43 2016 +0200
ipv4: Namespaceify tcp_keepalive_time sysctl knob
Different net namespaces might have different requirements as to
the keepalive time of tcp sockets. This might be required in cases
where different firewall rules are in place which require tcp
timeout sockets to be increased/decreased independently of the host.
Signed-off-by: Nikolay Borisov <email@example.com>
Signed-off-by: David S. Miller <firstname.lastname@example.org>