3.6.0 postgres: missing known db name

[etj]

The current docker-entrypoint.sh file checks the existence of the GeoNetwork DB by a select in pg_database with the line:

if psql -h "$db_host" -U "$POSTGRES_DB_USERNAME" -p "$db_port" -tqc "SELECT 1 FROM pg_database WHERE datname = '$db_name'" | grep -q 1; then

Anyway pgsql will try and connect to a DB with the same name of the user if a db is not specified.
In the case no DB exists with such a name, the script will misbehave.
We could use the default postgres DB for the test connection.