Tricky: docker, oraclelinux, ubuntu and ulimit -Hn

Virtualization is said to bee good for training and development, but  it should be kept out from productive environments. Some people, to whom I belong, meanwhile changed their minds and are convinced, that e.g. docker containers running ubuntu or oraclelinux, breed85/oracle12c or alike will play an increasing role in data virtualization. See also oaktable-world-agenda-2015/ To get experience and clarify certain questions we have created a Docker Project on github. This story here came up with the number of open files in a system (ulimit -Hn), crucial for productive systems. The issue discussion serves best to  understand the impact and the trickyness of the question. For whom it may concern, it is published here.