soooooo
after talking to a colleague, he mentioned that this is an issue in runc, that needs to be fixed in runc, then in docker, then all is good. See https://github.com/opencontainers/runc/pull/1184
first things first: This will just spill a non-fatal message, flooding the logs, but everything should still work.
However you could start the ES node with a system property -Des.cgroups.hierarchy.override=/ (note, system property, not ES parameter), to make this work again. Not that this is an undocumented parameter, that could be removed or changed over time.
Hope this helps.
--Alex