build: create new eXist-db directory for ahikar-test

The index of the database at /exist/data is corrupt on the test instance but cannot be deleted. Until we can make a decision about that in the first week of October, we create a new bind mount, exist_2, to get the database running again.

As a result, http://ahikar-test.uni-goettingen.de is running again, and the old database is preserved in ./exist/data until we decide what to do with it.

Cf. #34 (closed).