bug or feature in oracle-rdbms-server-12cR1-preinstall?

The latest version of oracle-rdbms-server-12cR1-preinstall introduces an interesting behaviour: it blocks su - oracle with the vacuous message su: could not open session. But we really need to be able to do that, when we want to install Oracle12c.
There are many questions and answers in the net trying to get rid of that extremely annoying issue. Here is the solution.
My concrete situation was, that I intended to install Oracle12c in a docker image - breed85/oracle-12c:preinstall, to be precise - which comes along with the user oracle already created. I checked as root  su - oracle, and there was no problem.

Next step was to execute yum -y install oracle-rdbms-server-12cR1-preinstall. I checked as root again su - oracle and there was a problem. By the way: that happened with other suitable docker images like oracle/oraclelinux, too. So the image was not the culprit. Most baffling was, that I had executed the oracle-rdbms-server-12cR1-preinstall some weeks ago and did not encounter any problem.

To cut edges - the reason were missing lines in /etc/security/limits.conf. The missing lines are contained in

oracle-rdbms-server-12cR1-preinstall

You need just attach the lines to the limits.conf like that

insert missing lines

and you can continue to install.