jitsi meet/mumble (here is web interface too)/owncasy

In your case you can try this:

$ docker exec -ti lemmy_postgresql_1 bash

or put here your container name instead "lemmy_postgresql_1", look inside docker ps output

$ su postgres 
$ psql -d lemmy
$ UPDATE local_site SET private_instance = false;

Then restart docker container with docker restart lemmy_lemmy_1 # or your container name instead "lemmy_lemmy_1", look inside docker ps output

Connect to your database and execute this sql:

UPDATE local_site SET private_instance = false

Then restart docker container