(MAINT) Add root directory for persistence

[nwolfe]

Just to help keep things clean and separated, it's nice to have a root
home for this kind of thing to make it easy to rm-rf, gitignore,
reference, etc.

[nwolfe]

This makes it easier for me to play around with these services and compose file.

Haven't considered production impact. Figured I might as well put up a PR for discussion.

[underscorgan]

I was thinking about this recently when I accidentally deleted some WIP trying to reset my server data...I'm definitely 👍 on this, though it will be breaking. I'm not sure the best way to go about that

[lutter]

I am all for this; and I think the sooner we merge it, the less breakage we'll have.

Personally, I'd prefer a name like data or state or volumes over persistence because the latter makes me think it's got code to do with persistence in it.

[nwolfe]

Yep I agree, volumes is a much better name for this directory.

Turns out there is some prior art in the community around special "migration" containers in your compose file, which I came across in this issue: docker/compose#2033

I haven't dug in, but I think the idea is to have your first container's entrypoint script check for the old state and then migrate it if necessary before running the actual container entrypoint. This could happen either in a special migration container that is depended on by the real service container, or baked into the service container's entrypoint script itself.

Not sure if we want to play around with a migration approach like this just yet though, or even attempt to deal with it at all.

[lutter]

I took the liberty to update this PR by doing a s/persistence/volumes. With that, I think it's good to merge.