Container registry metadata database: migration on docker installation

I am currently running Gitlab enterprise edition with all-in-one docker image, (gitlab/gitlab-ee) and enabled Container Registry before the availability of container registry metadata database.

With this image, a postgresql instance is run inside the container.

I am now having trouble migrating to the container registry metadata database. As Container registry metadata database | GitLab mentioned, I need to append something to gitlab.rb (despite I am currently passing configurations by GITLAB_OMNIBUS_CONFIG envvar, it is ok to use gitlab.rb) like below:

registry['database'] = {
  'enabled' => false, # Must be false!
  'host' => 'localhost',
  'port' => 5432,
  'user' => 'registry-database-user',
  'password' => 'registry-database-password',
  'dbname' => 'registry-database-name'
  'sslmode' => 'require', # See the PostgreSQL documentation for additional information https://www.postgresql.org/docs/current/libpq-ssl.html.
  'sslcert' => '/path/to/cert.pem',
  'sslkey' => '/path/to/private.key',
  'sslrootcert' => '/path/to/ca.pem'
}

But since I was on the all-in-one docker installation, I have no idea about the database name, db username or password to be filled in.

Any idea about this scenario?
Thanks.

+1 on this topic.