docker: reorganized the README, same infos
docker: reorganized the README, same infos

This is in preparation for the next commit, which will introduce support for
custom builds with plugins.

aptivate/etherpad-lite,ether/etherpad-lite,aptivate/etherpad-lite,storytouch/etherpad-lite,ether/etherpad-lite,aptivate/etherpad-lite,joequant/etherpad-lite,joequant/etherpad-lite,aptivate/etherpad-lite,joequant/etherpad-lite,storytouch/etherpad-lite,joequant/etherpad-lite,storytouch/etherpad-lite,ether/etherpad-lite