Move default data bind mount from system directory to local directory

[spwoodcock]

See https://forum.getodk.org/t/odk-central-make-data-directory-configurable/44760/5

What has been done to verify that this works as intended?

I can start the compose stack without issue.

Why is this the best possible solution? Were any other approaches considered?

This offers flexibility of user configuration, plus a sane default.

How does this change affect users? Describe intentional changes to behavior and behavior that could have accidentally been affected by code changes. In other words, what are the regression risks?

Any users with data located under /data/transfer will have to either:

1. Update their .env with DATA_DIR=/data/transfer.
2. Migrate the data to ./data/transfer, relative to the git repo.
3. Move the data anywhere else on their system and set DATA_DIR accordingly.

Does this change require updates to documentation? If so, please file an issue here and include the link below.

Yes, I will PR the docs after this is approved.

Before submitting this PR, please make sure you have:

• branched off and targeted the next branch OR only changed documentation/infrastructure (master is stable and used in production)
• verified that any code or assets from external sources are properly credited in comments or that everything is internally sourced