The default Discourse's `db_shared_buffers` value is 256Mb

Administration
1 
cd <Discourse folder>
./launcher enter <container>
su - postgres
psql -c "SELECT setting FROM pg_settings WHERE 'shared_buffers' = name;"

04

32768 * 8192 / (1024 * 1024) = 256.

dba.stackexchange.com/a/103778