postgres - c  =  not working for postgres 11 in docker
You cannot put spaces around the equals sign when using the "-c" option.  So you need something like postgres -c max_connections=200

When I try it with the spaces, it refused to start at all.  So I don't know what you are connecting to in order to SHOW max_connections;.  Maybe you already had a server running before you tried to start with the invalid syntax?