Per the documentation for
"Requires the CONTROL permission on the database, or ALTER ANY DATABASE permission, or membership in the db_owner fixed database role."
A possible way to handle this is to add the user to the model database as part of the
db_owner role. Every database created in SQL Server is a copy of that and will retain all users, objects, grants, etc. This means that whenever a new database is created, it will retain your CI user in the db_owner role and have permission to drop/alter that database. The drawback, of course, is that this will carry over to databases not created as part of your CI process as well. You would have to go in and manually remove that if you didn't want this.
Otherwise, your CI login will need high enough permissions to create and drop any database, as the hurdles to restrict this start to become extremely complex.