BTW; I had to change the PostgreSQLContainerType to:
class PostgreSQLContainerType[SELF](dockerImageName: String) extends PostgreSQLContainer[PostgreSQLContainerType[SELF]](dockerImageName)
The use-case in JAVA is to allow builder-pattern (having the with*()-methods implemented in abstract super-class), and return the concrete class:
new PostgreSQLContainerType[PostgreSQLContainerType[_]]("postgres:12.1")
.withDatabaseName("XXX").withUsername("XXX").withPassword("XXX")