Reference taken from this blog:
Script to Create Read-Only user:
CREATE ROLE Read_Only_User WITH LOGIN PASSWORD 'Test1234'
NOSUPERUSER INHERIT NOCREATEDB NOCREATEROLE NOREPLICATION VALID UNTIL 'infinity';
Assign permission to this read only user:
GRANT CONNECT ON DATABASE YourDatabaseName TO Read_Only_User;
GRANT USAGE ON SCHEMA public TO Read_Only_User;
GRANT SELECT ON ALL TABLES IN SCHEMA public TO Read_Only_User;
GRANT SELECT ON ALL SEQUENCES IN SCHEMA public TO Read_Only_User;
Assign permissions to read all newly tables created in the future
ALTER DEFAULT PRIVILEGES IN SCHEMA public GRANT SELECT ON TABLES TO Read_Only_User;