You install EGroupware first. So all the containers will be installed as well.
The EGroupware data is stored in the DB on the host and in /var/lib/egroupware (backups, VFS data, …). Now you can do a migration like before 19.1. Take a look here:
Make a current backup, copy the files and restore the backup.
Oh, ist that your developer box?
Only with the Rocketchat DB (Mongo) I am unsure. There @RalfBecker would have to say something.
There would still have to be some documentation. The backup too…
There is a
/var/lib/egroupware/default/rocketchat/dump . For Backup?