Add documentation about the missing Oracle Docker image

See jhipster/generator-jhipster#9370

pages/using_oracle.md

@@ -26,3 +26,4 @@ When using Oracle with JHipster, the following limitations will be applicable
 - When doing relationships, foreign key names cannot be more than 30 characters, so they will be truncated if they are too long
 - When doing many-to-many relationships, the join table name will follow the JPA specification (in the form "firstTable_secondTable"): if it is more than 30 characters long, it will be truncated.
 - Oracle reserved keywords cannot be used as Entity names or Field names.
+- We do not provide an Oracle database Docker image, like we do for other databases, as Oracle does not allow to have public Docker images.