Migrate MySQL to Oracle using multiple virtualboxes

After 10 years I have come back to MySQL and there is quite a lot of things which may concern an Oracle DBA.

MySQL and Oracle live under the same company roof
as I have predicted 12 years ago and the results of the meanwhile peaceful  conjugal live have turned out to be impressing for Oracle DBA's as well.

Oracle's SQLDeveloper and Vagrant

Refers to github project train.oracle12.migration

The Problem

You want to run your SQLDeveloper on the host and you want to let it work on the databases inside the guest VM.  How do we tell SQLDeveloper, that it should connect to the databases inside the guest VM ?