database_examples uses jOOQ code generation, so it needs a running PostgreSQL during build. The project also does some integration tests agains another PostgreSQL database running on docker which must be installed.
You can either setup PostgreSQL natively on your machine, create a database user spring with the same password having access to database pagila or use the Maven to fire up an instance on docker with
mvn docker:start
The project is ready configured to run agains this database.
Build it with
mvn clean verify