Jul 13 12:25:41 example.com dockerd[1390]: time="2018-07-13T12:25:41.208398606+01:00" level=warning msg="Your kernel does not support cgroup rt period"
Jul 13 12:25:41 example.com dockerd[1390]: time="2018-07-13T12:25:41.208718365+01:00" level=warning msg="Your kernel does not support cgroup rt runtime"

Linux example.com 4.13.0-45-generic #50~16.04.1-Ubuntu SMP Wed May 30 11:18:27 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux
 # docker --version
Docker version 18.03.1-ce, build 9ee9f40

139     cpuRealtimeRuntime := cgroupEnabled(mountPoint, "cpu.rt_runtime_us")
140     if !quiet && !cpuRealtimeRuntime {
141         logrus.Warn("Your kernel does not support cgroup rt runtime")
142     }

2.3 Basis for grouping tasks
----------------------------

Enabling CONFIG_RT_GROUP_SCHED lets you explicitly allocate real
CPU bandwidth to task groups.

This uses the cgroup virtual file system and "<cgroup>/cpu.rt_runtime_us"
to control the CPU time reserved for each control group.