We've had reports of the `:latest` tag not working anymore,
leading to an error as described here:
https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/341

This reverts commit 61e93744ec.

Reverting because of:
- https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/341
- https://github.com/matrix-org/matrix-appservice-slack/issues/347

NeDB is deprecated, so we should transition away from it in the future.
Still, most of the functionality should work on this new version.