Test/document cgroups behavior

[olix0r]

Linux cgroups(7) can limit cpu shares for a process (typically via Docker). Processes that use num_cpus, however, may be shown the number of CPUs available on the host OS and not the CPU shares as provided by cgroups.

I'm not 100% sure of the state of things, but we should build a small dockerized demo application to validate that CPU limits are reflected through num_cpus. If it's not cgroups-aware, we should make it so.

[lucab]

Relevant related logic from OpenJDK 15: https://github.com/openjdk/jdk/blob/jdk-15%2B0/src/hotspot/os/linux/os_linux.cpp#L5391-L5403