docker/assets/setup ignores uid overrides defined in gitlab.rb

Similar to #1157 (closed) and #1998 setup is static and ignores your own documentation. Specifically

user['uid'] = 
postgresql['uid'] =
redis['uid'] = 
web_server['uid'] = 

Implementation Guide

Add a troubleshooting section to specify that changing the uid/guid is not supported in Docker installations as the group ids are hardcoded during installation time.